On Windows, `PathUtility.GetRelativePath` was not properly handling
paths that differed by case in the drive reference (e.g. "C:\" vs.
"c:\"). The fix was to add the missing case-insensitive comparison
argument.
Replaced uses of `PathUtility.GetRelativePath` with
`Path.GetRelativePath` where possible (requires 2.0.0+).
Additionally, `PathUtility.RemoveExtraPathSeparators` was not handling
paths with drive references on Windows. If the path contained a drive
reference, the separator between the drive reference and the first part
of the path was removed. This is due to `Path.Combine` not handling
this case, so an explicit concatenation of the separator was added.
This commit resolves issue #7699.
