Stabilize Python upgrades (#17766)
Includes a few things...
- Drops preview warnings for use of `uv python upgrade` and `uv python
install --upgrade`
- Adds `--resolve-links` to `uv python find`, which I needed in test
cases to retain existing snapshots
- Fixes issues in our "Using environment ..." messages on Windows which
were incorrect
- Refactors `from_executable` for the `PythonMinorVersionLink` type
(https://github.com/astral-sh/uv/pull/17842/commits/28b2ed2525327d94fdf5372a29bbbc476d74680f)
to use the type system to prevent incorrect construction (for above)
- Removes special casing where we only upgrade links if they already
exist, which existed so preview wasn't needed on every invocation
- Fixes a bug with `PythonMinorVersionLink::exists` which returned
`true` even if the link pointed to the wrong Python installation leading
to discovery failures
