Update usages of ! is with is not (#2584)

* Update usages of ! is with is not

* Content.IntegrationTests commit

* Content.Server commit

* Content.Shared commit

Co-authored-by: Metal Gear Sloth <metalgearsloth@gmail.com>

Content.Client/GameObjects/EntitySystems/VerbSystem.cs

@@ -142,7 +142,7 @@ namespace Content.Client.GameObjects.EntitySystems
                 return true;
             }
 
-            if (!(_stateManager.CurrentState is GameScreenBase gameScreen))
+            if (_stateManager.CurrentState is not GameScreenBase)
             {
                 return false;
             }