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/Style.cs

@@ -165,10 +165,10 @@ namespace Spectre.Console
         {
             int? GetLinkHashCode()
             {
-#if NET5_0
-                return Link?.GetHashCode(StringComparison.Ordinal);
-#else
+#if NETSTANDARD2_0
                 return Link?.GetHashCode();
+#else
+                return Link?.GetHashCode(StringComparison.Ordinal);
 #endif
             }