Remove `--target-version=py38` from `ruff check` and `ruff format`

[ohmayr]

We've added --target-version=py38 to format and sort our generated code so that the format is compatible with the min supported Python version which at this time is python 3.8. Without this, we have failing unit tests and lint presubmits for 3.8.

Remove --target-version=py38 from nox sessions using ruff to format and sort code with the gapic-generator, once support from Python 3.8 is dropped. The code locations can be looked up by searching for this issue url.