Future-proof conditional compilation

* Invert `#if NET5_0` conditions so that when adding net6.0 target framework, the _new_ APIs are used.
* Use `NET5_0_OR_GREATER` instead of `NET5_0` to ensure consistent behaviour on future target frameworks.

src/Spectre.Console/Internal/Backends/Ansi/AnsiLinkHasher.cs

@@ -32,10 +32,10 @@ namespace Spectre.Console
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static int GetLinkHashCode(string link)
         {
-#if NET5_0
-            return link.GetHashCode(StringComparison.Ordinal);
-#else
+#if NETSTANDARD2_0
             return link.GetHashCode();
+#else
+            return link.GetHashCode(StringComparison.Ordinal);
 #endif
         }
     }