Update min Python version to 3.10 (in 22.04)

[benhoyt]

We've decided we'll do this after 31 May 2025 (standard EOL date for 20.04). So we'll drop support for Python 3.8 one release after that. We'll switch to Python 3.10 in the June Ops release, however, we announce it right away, and in the May release notes.

Charms that don't need 20.04 support will just be able to upgrade. Charms that need 20.04 support will have to create a maintenance branch.

[benhoyt]

This is "almost" ops-next.

[dimaqq]

re docs: https://docs.readthedocs.com/platform/stable/tutorial/index.html#setting-stable-as-the-default

[dwilding]

I think we'll still want latest as the default (essentially ignoring stable). As I understand it, stable is intended to point to the most recent versioned release, i.e., 3.x, whereas latest points to whatever is in main. We decided that we don't want to differentiate between release and pre-release docs, which means we should continue with latest as the default.

[tonyandrewmeyer]

Done in #1873