Update LangVersion to C# 12.

Now that we have moved to net 8 from net 6, and with us no longer supporting the mono runtime, we can raise the C# version to match that supported by the net 8 runtime.

This unlocks C# 10, C# 11 and C# 12 features previously unavailable to us.
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-10
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-11
- https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-version-history#c-version-12

A newer version of StyleCop is required to avoid rules tripping up on the new syntax.

Enable a handful of style rules that don't have many violations.

OpenRA.Game/Scripting/ScriptContext.cs

@@ -204,7 +204,7 @@ namespace OpenRA.Scripting
 
 			runtime.Globals["MaxUserScriptInstructions"] = MaxUserScriptInstructions;
 
-			using (var fn = runtime.CreateFunctionFromDelegate((Action<string>)LogDebugMessage))
+			using (var fn = runtime.CreateFunctionFromDelegate(LogDebugMessage))
 				runtime.Globals["print"] = fn;
 
 			// Register global tables