Document and establish new MSRV policy

[MichaReiser]

• Update Ruff's versioning policy to allow MSRV bumps in patch versions
• Decide on whether we want to use latest - 1 or latest - 2 (future bumps should happen as part or shortly after bumping the rust toolchain version)
• Bump the MSRV accordingly

[MichaReiser]

@carljm do you want to pick this up as you seem most excited about a more recent MSRV?

[carljm]

Sure, I can do this. I checked off the "decide on N-1 vs N-2" task above, since I think we've agreed on N-2.

[charliermarsh]

I think this got done in the linked PRs?