Modify cache versioning to support backwards compatibility (#8386)
## Summary
Going forward, we're going to provide better versioning guarantees
around using the same cache across multiple uv versions, so this PR
updates the docs to reflect that. It also bumps the `sdists-` version to
fix the inconvenience demonstrated in
https://github.com/astral-sh/uv/issues/8367.
Closes https://github.com/astral-sh/uv/issues/8367.
