4.4.0 Resilient data update

Flag data are now saved on Entity.
This makes flags removable even after server crashes or after the plugin data file have been deleted.
Flags which data can't be saved because of a crash won't be removable by `/fh remove [id]` nor show in the flag list. But they still can be destroyed by player hand.
Most servers won't be affected by this update, but the update will benefit to the server that crashes some time.

Explicitly support 1.21.4.