bsd.port.mk: Use zstandard as default compression for FreeBSD 14+

The packages are switched from XZ to ZSTD compression, the compression level
is set at the maximum possible because it creates packages which are closed
in size between XZ and ZSTD, the compression time is close as well,
but decompression time is way bigger.

Exp-run: antoine
Differential revision: https://reviews.freebsd.org/D35095