Fix relative path handling on Windows.

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.

src/dotnet/commands/dotnet-sln/remove/Program.cs

@@ -41,7 +41,7 @@ namespace Microsoft.DotNet.Tools.Sln.Remove
             SlnFile slnFile = SlnFileFactory.CreateFromFileOrDirectory(_fileOrDirectory);
 
             var relativeProjectPaths = _appliedCommand.Arguments.Select(p =>
-                                                                            PathUtility.GetRelativePath(
+                                                                            Path.GetRelativePath(
                                                                                 PathUtility.EnsureTrailingSlash(slnFile.BaseDirectory),
                                                                                 Path.GetFullPath(p)))
                                                       .ToList();