Per mob mob_griefing loot pickup override

[Peashooter101]

Resolves #1739 which asks for more granular control of mob_griefing loot pickup overrides.

There were some mob griefing overrides that already existed for some mobs such as the piglin.

• If mob griefing overrides existed for a mob already, we convert that setting to the new one. For example, mobs.piglin.mob-griefing-override becomes mobs.piglin.can-pick-up-loot.
• All other mobs without a pre-existing mob_griefing setting that only handles item pickup pull from gameplay-mechanics.entities-pick-up-loot-mob-griefing-override and convert it to mobs.<mob>.can-pick-up-loot.

Post Code Review Tasks

• Update documentation