Fork when minimum Python version increases (#8628)
## Summary
This is a re-implementation of
https://github.com/astral-sh/uv/pull/4712, though is now seemingly much
simpler. This issue keeps coming up, and users have a workaround with
`tool.uv.environments`, but it's really a bug in the resolver.
Closes https://github.com/astral-sh/uv/issues/4668.
