path/filepath: Rel returns non-Clean'ed path. [1.24 backport]

[gopherbot]

@kolyshkin requested issue #75763 to be considered for backport to the next 1.24 minor release.

I think this needs to be backported to all supported releases.

@gopherbot please consider this for backport to 1.25 and 1.24.

[gopherbot]

Change https://go.dev/cl/710456 mentions this issue: [release-branch.go1.24] path/filepath: return cleaned path from Rel

[dmitshur]

Asked for a more detailed backport rationale in #75763 (comment).