Use lower-bound semantics for all Python compatibility comparisons (#6882)
## Summary
Right now, we have slightly different `requires-python` semantics for
`-p 3.11` vs. `-p 3.11 --universal`, and slightly different (wrong)
semantics for how we compare against the _installed_ Python version
(which doesn't ignore upper bounds, but should).
This PR rips it all out and replaces it with consistent semantics across
`uv lock`, `uv pip compile -p 3.11`, and `uv pip compile -p 3.11
--universal`. We now always ignore upper bounds.
Closes https://github.com/astral-sh/uv/issues/6859.
Closes https://github.com/astral-sh/uv/issues/5045.