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.Mods.Common/UtilityCommands/FuzzMapGeneratorCommand.cs

@@ -12,6 +12,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Globalization;
 using System.Linq;
 using System.Text;
 using OpenRA.Mods.Common.Traits;
@@ -237,7 +238,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 					var descriptionBuilder = new StringBuilder();
 					foreach (var variable in config.Variables)
-						descriptionBuilder.Append($"  {variable}={iterationChoices[variable]}\n");
+						descriptionBuilder.Append(CultureInfo.InvariantCulture, $"  {variable}={iterationChoices[variable]}\n");
 
 					if (!config.NoDefaults)
 						descriptionBuilder.Append("  (+Defaults)\n");