Backport MOB_CONVERSION event listener to 1.20 branch

[DaFuqs]

The new branches use the fabric MOB_CONVERSION event to copy entity data to the new entity.
The 1.20 branch does not make use of that event. As a result, when a mob converts it loses its CCA data in the process.

It would be nice having that for the 1.20 branch, too.

[Pyrofab]

Could be nice, but some of the changes associated with this feature are breaking changes, so those couldn't be backported. Additionally I don't have much time to spend on backports, but if you or someone else submitted a PR with a backward-compatible version of this change I could release it.