Project mips as unsupported for 14.x.

While MIPS is still present in main, the consensus in recent
discussions on arch@ and mips@ is to remove it prior to 14.0.

https://lists.freebsd.org/archives/freebsd-arch/2021-September/000068.html
https://lists.freebsd.org/archives/freebsd-mips/2021-September/000014.html

Reviewed by: imp, emaste
Differential Revision: https://reviews.freebsd.org/D32263