Random spontaneous cleanup PR (#25131)

* Use new Subs.CVar helper

Removes manual config OnValueChanged calls, removes need to remember to manually unsubscribe.

This both reduces boilerplate and fixes many issues where subscriptions weren't removed on entity system shutdown.

* Fix a bunch of warnings

* More warning fixes

* Use new DateTime serializer to get rid of ISerializationHooks in changelog code.

* Get rid of some more ISerializationHooks for enums

* And a little more

* Apply suggestions from code review

Co-authored-by: 0x6273 <0x40@keemail.me>

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Co-authored-by: 0x6273 <0x40@keemail.me>

Content.Shared/Hands/EntitySystems/SharedHandsSystem.cs

@@ -18,7 +18,6 @@ public abstract partial class SharedHandsSystem
     [Dependency] private readonly ActionBlockerSystem _actionBlocker = default!;
     [Dependency] protected readonly SharedContainerSystem ContainerSystem = default!;
     [Dependency] private readonly SharedInteractionSystem _interactionSystem = default!;
-    [Dependency] private readonly SharedItemSystem _items = default!;
     [Dependency] private readonly SharedStorageSystem _storage = default!;
     [Dependency] protected readonly SharedTransformSystem TransformSystem = default!;
     [Dependency] private readonly SharedVirtualItemSystem _virtualSystem = default!;