TST: Check new astropy file overwrite error message in logging test

[rrjbca]

Description

Use new NOT_OVERWRITING_MSG from astropy when checking for file overwrite error message in logging test. Fallback on old error message for previous versions of astropy where it is unavailable. Tested locally using tox -v -e py39-test and tox -v -e py39-test-dev so should resolve current compatibility workflow failure. Resolves #497

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [x] Write documentation strings
• [x] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request