ci: stop building on freebsd-12-1

ci: stop building on freebsd-12-1

An updated freebsd-12-2 image was added a few months ago, and this
older one is consistently failing to go past `pkginstall`:
```
Newer FreeBSD version for package py37-mlt:
To ignore this error set IGNORE_OSVERSION=yes
- package: 1202000
- running kernel: 1201000
Ignore the mismatch and continue? [Y/n]: pkg: repository FreeBSD contains packages for wrong OS version: FreeBSD:12:amd64
```

FreeBSD thread suggests that 12.1 is EOL, and best to avoid.

Ref: https://forums.freebsd.org/threads/78856/

Reviewed-by: Daniel Stenberg
Closes #6622