No datafixer for item AttributeModifiers UUIDs

The bug

When upgrading from 1.15.2 or 20w11a to 20w12a, item attribute modifiers are not datafixed to the new UUID format, rendering them broken.

To reproduce

In 20w11a:

/give @p stone_sword{AttributeModifiers:[{Name:"generic.attackDamage",Operation:0,Amount:50,Slot:"mainhand",UUIDMost:1L,UUIDLeast:2L}]}

Upgrade to 20w12a and check the sword: it no longer has extra attack damage because the NBT wasn't upgraded