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/Support/HttpQueryBuilder.cs

@@ -12,6 +12,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Text;
 
 namespace OpenRA.Support
@@ -40,7 +41,7 @@ namespace OpenRA.Support
 			builder.Append('?');
 
 			foreach (var parameter in parameters)
-				builder.Append($"{parameter.Key}={parameter.Value}&");
+				builder.Append(CultureInfo.InvariantCulture, $"{parameter.Key}={parameter.Value}&");
 
 			return builder.ToString();
 		}