Update Python version in snaps before October

[bmw]

In October, Python 3.8 will reach its end-of-life (see https://devguide.python.org/versions/#supported-versions). Before that happens, I think we should migrate our snaps to a newer version of Python. I'd recommend using Ubuntu 24.04 as a base and whatever Python version ships with it but we could start testing approaches with Ubuntu 22.04 until 24.04 is out.

We have never done this before. We also didn't seem to formulate an exact plan for how we're going to do this. The best I could find is https://docs.google.com/document/d/1OB5WCDz2mBMgRvtnvo4FSjWHPBDlt1NpccJ5sNrhWIw/edit, but others are welcome to go spelunking through our GitHub and Google Docs like I did to see if there's anything we missed. It's also possible Erica remembers something that I do not here.

Resolving this will like require a fair bit of testing, research, design work, and coordination with authors of 3rd party plugin snaps.