[v2 Bug] Jinja file extensions not working

### Is this a new bug in dbt v2.x compared to the latest version of dbt 1.x?

- [x] I believe this is a new bug in dbt v2.x
- [x] I have searched the existing issues and could not find a duplicate

### Current Behavior

This [feature](https://docs.getdbt.com/reference/global-configs/behavior-flags/allow_jinja_file_extensions) seems to be missing from V2.

### Expected Behavior

When a sql file is suffixed with `j2` and `allow_jinja_file_extensions` is set to true, the file should be recognised expanded as a Jinja file. That doesn't happen.

### Steps To Reproduce

1. Add a file with `foo.sql.j2`
2. In `dbt_project.yml` add
```
flags:
allow_jinja_file_extensions: true
```
3. Run

### Relevant log output

```shell

```

### Environment

```markdown
- OS: MacOS
- CPU: ARM
- dbt distribution and version: 2.0.0-preview.218
```

### Which database adapter are you using?

_No response_

### Is this a discrepancy vs. dbt 1.x?

- [x] Yes — this works in dbt 1.x but not in dbt v2.x

### Additional Context

_No response_