[1.20.4] Alter StartTracking with Before and After Variants

[Dragon-Seeker]

The Point of this PR is to have an After Variant for StartTracking as the current implementation calls the StartTracking event before any packets for the Entity's creation making such an event pointless if you need to sync things like data attachment to the Client as such will require some delayed method of handling any packet sent until the entity exists. Evidence of such an event being useful can be found within Cardinal Components and NeoForge.

Additionally, this PR attempts to keep binary compatibility with previous API versions with such old variants extending the renaming of the old event.