Paper 1.21.11 crash fix - 2.1.7 broke older servers.

Fixed

• Server crash when a bot takes knockback on 1.21.x - 2.1.7 wrote the 26.3-only syncVelocity field directly, which threw NoSuchFieldError and crashed the world tick. The flag is now set by reflection, trying syncVelocity then hurtMarked.
• Bot block clicks silently failing on 1.21.x - the accept-teleport packet is now built with whichever constructor the server has (id only on 1.21.x, id + position + rotation on 26.3), so bots can use items on blocks again after a teleport.