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/Exts.cs

@@ -482,7 +482,7 @@ namespace OpenRA
 				var badKeysFormatted = new StringBuilder(
 					$"{debugName}, duplicate values found for the following keys: ");
 				foreach (var p in dupKeys)
-					badKeysFormatted.Append($"{logKey(p.Key)}: [{string.Join(",", p.Value)}]");
+					badKeysFormatted.Append(CultureInfo.InvariantCulture, $"{logKey(p.Key)}: [{string.Join(",", p.Value)}]");
 				throw new ArgumentException(badKeysFormatted.ToString());
 			}
 		}