Seal/abstract/virtual everything (#6739)

Content.Shared/Chemistry/ReactiveSystem.cs

@@ -13,7 +13,7 @@ using Robust.Shared.Random;
 namespace Content.Shared.Chemistry
 {
     [UsedImplicitly]
-    public class ReactiveSystem : EntitySystem
+    public sealed class ReactiveSystem : EntitySystem
     {
         [Dependency] private readonly IPrototypeManager _prototypeManager = default!;
         [Dependency] private readonly IRobustRandom _robustRandom = default!;