Remove Python 3.8 as option; update default versions

[olivialynn]

Change Description

Originally just a PR to:

• Update marker for python version 3.13 in the docs (it was already a default, updating docs to reflect this)

However:

• Versions 3.8, 3.9 are pretty old by now
• https://devguide.python.org/versions/ shows 3.9's end of life was a couple weeks ago (Halloween)
• So, proposing we update the versions we suggest in the copier questions
• Going with conservative choice (just removing 3.8, and marking 3.9 EOL) but not against alternatives

Update:

• Adding in 3.14
• This closes Add Python 3.14 #548

Checklist

• This PR is meant for the lincc-frameworks/python-project-template repo and not a downstream one instead.
• This change is linked to an open issue
• This change includes integration testing, or is small enough to be covered by existing tests

[delucchi-cmu]

Love it.

While you're here, can you please add 3.14?

#548