az policy assignment issues

### **This is autogenerated. Please review and update as needed.**

## Describe the bug

**Command Name**
`a
z policy assignment create`

**Errors:**
```
APIVersion 2019-09-01 is not available
Traceback (most recent call last):
python3/dist-packages/knack/cli.py, ln 206, in invoke
cmd_result = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 608, in execute
raise ex
cli/core/commands/__init__.py, ln 666, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 659, in _run_job
six.reraise(*sys.exc_info())
...
cli/command_modules/resource/custom.py, ln 632, in _get_custom_or_builtin_policy
policy_operations = client.policy_set_definitions if for_policy_set else client.policy_definitions
mgmt/resource/policy/_policy_client.py, ln 175, in policy_definitions
raise NotImplementedError("APIVersion {} is not available".format(api_version))
NotImplementedError: APIVersion 2019-09-01 is not available
```

## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az policy assignment create --name {} --display-name {} --scope {} --policy {}`

## Expected Behavior

## Environment Summary
```
Linux-5.14.0-1051-oem-x86_64-with-glibc2.29
Python 3.8.10
Shell: bash

azure-cli 2.0.81

```
## Additional Context