Drop support for Python 3.8

[mikealfare]

Problem

Python 3.8 reached end of life on 10/7/2024.

Solution

Drop support for Python 3.8. Update tests to no longer run on Python 3.8.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-postgres contributing guide.