[freebsd] Update supported FreeBSD versions

[yasuhirokimura]

• Add FreeBSD 16
• Remove FreeBSD 12 (EoL on December 31, 2023)

[jiegec]

Thanks for you contributions! We are having some problems in FreeBSD CI, can you verify that it works on FreeBSD 16?

[yasuhirokimura]

Thanks for you contributions! We are having some problems in FreeBSD CI, can you verify that it works on FreeBSD 16?

I confirmed build succeeds on FreeBSD 16.0-CURRENT amd64 with following steps.

1. env FREEBSD_SYS=/usr/src/sys LSOF_OPINC="-idirafter /usr/src/sys" bash Configure -n freebsd
2. make