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.Cnc/UtilityCommands/ImportTiberianSunMapCommand.cs

@@ -10,6 +10,7 @@
 #endregion
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using OpenRA.Primitives;
 using OpenRA.Traits;
 
@@ -208,6 +209,7 @@ namespace OpenRA.Mods.Cnc.UtilityCommands
 			{ 0x65, DamageState.Undamaged },
 		};
 
+		[SuppressMessage("Style", "IDE0230:Use UTF-8 string literal", Justification = "False positive")]
 		protected override Dictionary<byte, byte[]> ResourceFromOverlay { get; } = new()
 		{
 			// "tib" - Regular Tiberium