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());
 			}
 		}

OpenRA.Game/FieldSaver.cs

@@ -95,7 +95,7 @@ namespace OpenRA
 					var formattedKey = FormatValue(key);
 					var formattedValue = FormatValue(value);
 
-					result.Append($"{formattedKey}: {formattedValue}{Environment.NewLine}");
+					result.Append(CultureInfo.InvariantCulture, $"{formattedKey}: {formattedValue}{Environment.NewLine}");
 				}
 
 				return result.ToString();

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

OpenRA.Game/Scripting/ScriptMemberWrapper.cs

@@ -100,7 +100,7 @@ namespace OpenRA.Scripting
 		public LuaValue Get(LuaRuntime runtime)
 		{
 			if (IsMethod)
-				return runtime.CreateFunctionFromDelegate((Func<LuaVararg, LuaValue>)Invoke);
+				return runtime.CreateFunctionFromDelegate(Invoke);
 
 			if (IsGetProperty)
 				return ((PropertyInfo)Member).GetValue(Target, null).ToLuaValue(context);

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();
 		}