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

Use Environment.OSVersion.Version.ToString() over .VersionString to get
a numercal 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())