Prefer higher Python lower-bounds when forking (#10007)
## Summary
With the advent of `--fork-strategy requires-python` (the default), we
actually _want_ to solve higher lower-bound forks before lower
lower-bound forks. The former ensures we get the most compatible
versions, while the latter ensures we get fewer overall versions. These
two strategies match up with `--fork-strategy`, but need to be respected
as such.
Closes https://github.com/astral-sh/uv/issues/9998.
