Deprecate / remove support for Python 3.8

[jeffwidman]

Code improvement description

Python 3.8 is now EOL'd by upstream:

• https://discuss.python.org/t/python-3-8-is-now-officially-eol/66983
• https://peps.python.org/pep-0569/
• https://devguide.python.org/versions/

So we should probably:

1. Pick a "drop" date when we officially drop support for it
2. Publish the deprecation blog post
3. Within dependabot-core add it to the Python deprecated versions so it triggers a warning on the jobs.
4. At some point once that deprecation date passes, we can actually delete it...

My personal opinion: I think we should move quickly to deprecate it with an aggressive "unsupported" date, but other than that it's okay if we're a little slow to actually delete the 3.8 code... just so long as we no longer guarantee support for it.

[jeffwidman]

I want to surface to the broader world that we're currently scheduling the work around this, so it will be landing in the next few weeks-to-months. So if you're still on 3.8, now's the time to start upgrading. 😄