platformutils: use Version.ToString() to get number only (#1264)

Use `Environment.OSVersion.Version.ToString()` over `.VersionString` to
get a numerical version only on Windows and macOS.

Using `.VersionString` on macOS you get values like "Unix 13.3.1" rather
than "13.3.1", and on Windows you get "Microsoft Windows NT 10.x.y.z"
rather than "10.x.y.z".

Author: mjcheetham

src/shared/Core/PlatformUtils.cs

@@ -372,7 +372,7 @@ private static string GetOSVersion(ITrace2 trace2)
 #endif
             if (IsWindows() || IsMacOS())
             {
-                return Environment.OSVersion.VersionString;
+                return Environment.OSVersion.Version.ToString();
             }
 
             if (IsLinux())