Macro `redshift_maintenance` errors if custom `ref()` macro exists in project

### Describe the bug
The `redshift_maintenance` macro throws a "Compilation Error" if a project has overridden the built in `ref()` macro ([example of how/why one might do that](https://docs.getdbt.com/reference/dbt-jinja-functions/builtins#usage)).

### Steps to reproduce
1. Any Redshift based dbt project.
2. Add this package to it.
3. Add a `ref()` macro (e.g. `macros/not_really_custom_ref.sql`) override that doesn't really do anything:
```jinja
{% macro ref(model_name) %}

{% set rel = builtins.ref(model_name) %}
{% set newrel = rel %}
{% do return(newrel) %}

{% endmacro %}
```
4. Try vacuuming via `dbt run-operation redshift_maintenance`.

### Expected results
Expected tables to be vacuumed and stdout to having something like:
```
22:21:04 Running with dbt=1.0.1
22:21:08 11:21:08 + 1 of 1 Vacuuming "analytics"."dbt_jyeo"."my_table"
22:21:08 11:21:08 + 1 of 1 Analyzing "analytics"."dbt_jyeo"."my_table"
22:21:09 11:21:09 + 1 of 1 Finished "analytics"."dbt_jyeo"."my_table" in 0.59s
```

_Note: above is the result of NOT having a custom `ref()` override._

### Actual results
A `dbt.exceptions.CompilationException` is raised instead:
```
22:36:12 Running with dbt=1.0.1
22:36:12 Change detected to override macro used during parsing. Starting full parse.
22:36:16 Encountered an error while running operation: Compilation Error in macro redshift_maintenance (macros/redshift_maintenance_operation.sql)
'dbt.clients.jinja.MacroGenerator object' has no attribute 'config'
```

### Screenshots and log output
Stdout with `--debug`:

Click to expand!

```shell

============================== 2022-02-13 22:36:54.114097 | 191d1b44-d1b6-4bc0-ad7c-59207c87bd30 ==============================
22:36:54.114097 [info ] [MainThread]: Running with dbt=1.0.1
22:36:54.115352 [debug] [MainThread]: running dbt with arguments Namespace(record_timing_info=None, debug=True, log_format=None, write_json=None, use_colors=None, printer_width=None, warn_error=None, version_check=None, partial_parse=None, single_threaded=False, use_experimental_parser=None, static_parser=None, profiles_dir='/Users/jeremy/.dbt', send_anonymous_usage_stats=None, fail_fast=None, event_buffer_size=None, project_dir=None, profile=None, target=None, vars='{}', log_cache_events=False, macro='redshift_maintenance', args='{}', defer=None, state=None, cls=, which='run-operation', rpc_method='run-operation')
22:36:54.115846 [debug] [MainThread]: Tracking: tracking
22:36:54.139491 [debug] [MainThread]: Sending event: {'category': 'dbt', 'action': 'invocation', 'label': 'start', 'context': [, , ]}
22:36:54.291982 [debug] [MainThread]: Partial parsing enabled: 0 files deleted, 0 files added, 0 files changed.
22:36:54.292516 [debug] [MainThread]: Partial parsing enabled, no changes found, skipping parsing
22:36:54.303169 [debug] [MainThread]: Sending event: {'category': 'dbt', 'action': 'load_project', 'label': '191d1b44-d1b6-4bc0-ad7c-59207c87bd30', 'context': []}
22:36:54.314694 [debug] [MainThread]: Acquiring new redshift connection "macro_redshift_maintenance"
22:36:54.315138 [debug] [MainThread]: Using redshift connection "macro_redshift_maintenance"
22:36:54.315434 [debug] [MainThread]: On macro_redshift_maintenance: BEGIN
22:36:54.315789 [debug] [MainThread]: Opening a new connection, currently in state init
22:36:54.316666 [debug] [MainThread]: Redshift adapter: Connecting to Redshift using 'database' credentials
22:36:56.453905 [debug] [MainThread]: SQL status: BEGIN in 2.14 seconds
22:36:56.454777 [debug] [MainThread]: On macro_redshift_maintenance: COMMIT
22:36:56.455268 [debug] [MainThread]: Using redshift connection "macro_redshift_maintenance"
22:36:56.455952 [debug] [MainThread]: On macro_redshift_maintenance: COMMIT
22:36:56.668484 [debug] [MainThread]: SQL status: COMMIT in 0.21 seconds
22:36:56.693942 [debug] [MainThread]: Postgres adapter: Error running SQL: macro redshift_maintenance
22:36:56.694449 [debug] [MainThread]: Postgres adapter: Rolling back transaction.
22:36:56.694901 [debug] [MainThread]: On macro_redshift_maintenance: Close
22:36:56.696044 [error] [MainThread]: Encountered an error while running operation: Compilation Error in macro redshift_maintenance (macros/redshift_maintenance_operation.sql)
'dbt.clients.jinja.MacroGenerator object' has no attribute 'config'
22:36:56.696788 [debug] [MainThread]:
Traceback (most recent call last):
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/clients/jinja.py", line 304, in exception_handler
yield
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/clients/jinja.py", line 259, in call_macro
return macro(*args, **kwargs)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/jinja2/runtime.py", line 675, in __call__
return self._invoke(arguments, autoescape)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/jinja2/runtime.py", line 679, in _invoke
rv = self._func(*arguments)
File "", line 23, in macro
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/jinja2/sandbox.py", line 407, in getattr
value = getattr(obj, attribute)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/jinja2/runtime.py", line 753, in __getattr__
return self._fail_with_undefined_error()
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/jinja2/runtime.py", line 747, in _fail_with_undefined_error
raise self._undefined_exception(self._undefined_message)
jinja2.exceptions.UndefinedError: 'dbt.clients.jinja.MacroGenerator object' has no attribute 'config'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/task/run_operation.py", line 57, in run
self._run_unsafe()
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/task/run_operation.py", line 44, in _run_unsafe
res = adapter.execute_macro(
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/adapters/base/impl.py", line 992, in execute_macro
result = macro_function(**kwargs)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/clients/jinja.py", line 332, in __call__
return self.call_macro(*args, **kwargs)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/clients/jinja.py", line 261, in call_macro
return e.value
File "/Users/jeremy/.pyenv/versions/3.9.7/lib/python3.9/contextlib.py", line 137, in __exit__
self.gen.throw(typ, value, traceback)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/clients/jinja.py", line 306, in exception_handler
raise_compiler_error(str(e), self.macro)
File "/Users/jeremy/src/dbt-sandcastles/venv_dbt_1.0.1/lib/python3.9/site-packages/dbt/exceptions.py", line 454, in raise_compiler_error
raise CompilationException(msg, node)
dbt.exceptions.CompilationException: Compilation Error in macro redshift_maintenance (macros/redshift_maintenance_operation.sql)
'dbt.clients.jinja.MacroGenerator object' has no attribute 'config'
22:36:56.709326 [debug] [MainThread]: Sending event: {'category': 'dbt', 'action': 'invocation', 'label': 'end', 'context': [, , ]}
22:36:56.711625 [debug] [MainThread]: Flushing usage events
22:36:57.600768 [debug] [MainThread]: Connection 'macro_redshift_maintenance' was properly closed.
```

### System information
**The contents of your `packages.yml` file:**
```yml
packages:
- package: dbt-labs/redshift
version: 0.6.0
```

**Which database are you using dbt with?**
- [x] redshift

**The output of `dbt --version`:**
```
installed version: 1.0.1
latest version: 1.0.1

Up to date!

Plugins:
- bigquery: 1.0.0
- snowflake: 1.0.0
- redshift: 1.0.0
- postgres: 1.0.1
- spark: 1.0.0
```

**The operating system you're using:**
```
ProductName: macOS
ProductVersion: 12.1
BuildVersion: 21C52
```

**The output of `python --version`:**
```
Python 3.9.7
```

### Additional context
This looks to be caused by the following 2 lines that try to get the root project name from the `ref.config` attribute - which is unavailable if a user manually overrides `ref` with their own.

https://github.com/dbt-labs/redshift/blob/48d6be2af1140c73bbba1ba5a6949225403cee43/macros/redshift_maintenance_operation.sql#L32-L33

As of [dbt 0.16](https://docs.getdbt.com/reference/dbt-jinja-functions/project_name), the `project_name` looks to be available without taking it from `ref.config` so we can change the above 2 lines to be something like:

```jinja
{% set root_project = context.project_name %}
{% if context.get(root_project, {}).get('vacuumable_tables_sql') %}
{% set vacuumable_tables_sql=context[root_project].vacuumable_tables_sql(**kwargs) %}
```

P.s. I did test the above code change and found that it worked - submitting PR after this issue :)

Start with macros/redshift_maintenance_operation.sql, especially the lines identified around 32–33, and reproduce the failure with a project that overrides ref(). Run dbt run-operation redshift_maintenance against the Redshift package and verify that maintenance completes without the reported CompilationException and produces the expected vacuum and analyze output.

Written by the indexing model from the issue text.