Seal/abstract/virtual everything (#6739)

Content.Server/Stack/StackComponent.cs

@@ -8,7 +8,7 @@ namespace Content.Server.Stack
     // TODO: Naming and presentation and such could use some improvement.
     [RegisterComponent, Friend(typeof(StackSystem))]
     [ComponentReference(typeof(SharedStackComponent))]
-    public class StackComponent : SharedStackComponent
+    public sealed class StackComponent : SharedStackComponent
     {
         [ViewVariables(VVAccess.ReadWrite)]
         public bool ThrowIndividually { get; set; } = false;

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!;