net/bird2: update 2.14 -> 2.15.1

- Port was updated to the most recent version.
- Extra patches for fixing netlink headers were prepared (see PR 277618).
- The default flavor of the port for FreeBSD 14.0 and later is now "netlink".
  The rtsock flavor is still available as bird2-rtsock.
  There is no change for FreeBSD versions < 14.0. Here we still use rtsock
  as a default flavor.

Changelog: https://bird.network.cz/pipermail/bird-users/2024-March/017468.html
	   https://bird.network.cz/pipermail/bird-users/2024-March/017517.html

PR: 277891