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.Client/GameTicking/Managers/ClientGameTicker.cs

@@ -38,7 +38,6 @@ namespace Content.Client.GameTicking.Managers
         [ViewVariables] public bool DisallowedLateJoin { get; private set; }
         [ViewVariables] public string? ServerInfoBlob { get; private set; }
         [ViewVariables] public TimeSpan StartTime { get; private set; }
-        [ViewVariables] public TimeSpan RoundStartTimeSpan { get; private set; }
         [ViewVariables] public new bool Paused { get; private set; }
 
         [ViewVariables] public IReadOnlyDictionary<NetEntity, Dictionary<string, uint?>> JobsAvailable => _jobsAvailable;