path/filepath: Clean removes ending slash for volume on Windows in Go 1.21.4 [1.21 backport]

[gopherbot]

@bcmills requested issue #64028 to be considered for backport to the next 1.21 minor release.

@gopherbot, please backport to Go 1.21 and maybe 1.20. This appears to be a regression introduced in a security release.

[gopherbot]

Change https://go.dev/cl/541521 mentions this issue: [release-branch.go1.21] path/filepath: consider \\?\c: as a volume on Windows

[gopherbot]

Change https://go.dev/cl/541837 mentions this issue: [release-branch.go1.21] path/filepath: consider \\?\c: as a volume on Windows

[gopherbot]

Closed by merging 8caf4bb to release-branch.go1.21.