Seal/abstract/virtual everything (#6739)

Content.Server/Stack/StackSystem.cs

@@ -21,7 +21,7 @@ namespace Content.Server.Stack
     ///     This is a good example for learning how to code in an ECS manner.
     /// </summary>
     [UsedImplicitly]
-    public class StackSystem : SharedStackSystem
+    public sealed class StackSystem : SharedStackSystem
     {
         [Dependency] private readonly IPrototypeManager _prototypeManager = default!;
         [Dependency] private readonly PopupSystem _popupSystem = default!;