Publish: Hint at `--skip-existing` -> `--check-url` transition (#8803)
See https://github.com/astral-sh/uv/pull/8531#issuecomment-2442698889,
we hint users coming from twine to use `--check-url` instead.
> `uv publish` does not support `--skip-existing`, use `--check-url`
with the simple index URL instead.
---------
Co-authored-by: Zanie Blue <contact@zanie.dev>