Add new .NET 9 rules to editorconfig.

Don't enforce all the rules yet, since we are still targeting .NET 6.

Adjust cast to nullable in UnloadCargo.ChooseExitSubCell. Avoid this cast tripping up CA2021 on older NET SDK versions.

OpenRA.Mods.Common/Activities/UnloadCargo.cs

@@ -55,8 +55,7 @@ namespace OpenRA.Mods.Common.Activities
 
 			return cargo.CurrentAdjacentCells()
 				.Shuffle(self.World.SharedRandom)
-				.Select(c => (c, pos.GetAvailableSubCell(c)))
-				.Cast<(CPos, SubCell SubCell)?>()
+				.Select(c => ((CPos Cell, SubCell SubCell)?)(c, pos.GetAvailableSubCell(c)))
 				.FirstOrDefault(s => s.Value.SubCell != SubCell.Invalid);
 		}