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");

OpenRA.Mods.Common/Widgets/Logic/Ingame/GameInfoLogic.cs

@@ -157,8 +157,8 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 		{
 			Game.LoadWidget(world, "LOBBY_OPTIONS_PANEL", optionsPanelContainer, new WidgetArgs()
 			{
-				{ "getMap", (Func<MapPreview>)(() => modData.MapCache[world.Map.Uid]) },
-				{ "configurationDisabled", (Func<bool>)(() => true) }
+				{ "getMap", () => modData.MapCache[world.Map.Uid] },
+				{ "configurationDisabled", () => true }
 			});
 		}
 

OpenRA.Mods.Common/Widgets/Logic/Lobby/LobbyLogic.cs

@@ -189,8 +189,8 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 					"onMouseDown", (Action<MapPreviewWidget, MapPreview, MouseInput>)((preview, mapPreview, mi) =>
 						LobbyUtils.SelectSpawnPoint(orderManager, preview, mapPreview, mi))
 				},
-				{ "getSpawnOccupants", (Func<Dictionary<int, SpawnOccupant>>)(() => spawnOccupants) },
-				{ "getDisabledSpawnPoints", (Func<HashSet<int>>)(() => orderManager.LobbyInfo.DisabledSpawnPoints) },
+				{ "getSpawnOccupants", () => spawnOccupants },
+				{ "getDisabledSpawnPoints", () => orderManager.LobbyInfo.DisabledSpawnPoints },
 				{ "showUnoccupiedSpawnpoints", true },
 				{ "mapUpdatesEnabled", true },
 				{
@@ -370,7 +370,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 			var optionsBin = Ui.LoadWidget("LOBBY_OPTIONS_BIN", lobby.Get("TOP_PANELS_ROOT"), new WidgetArgs()
 			{
 				{ "orderManager", orderManager },
-				{ "getMap", (Func<MapPreview>)(() => map) },
+				{ "getMap", () => map },
 				{ "configurationDisabled", configurationDisabled }
 			});