Mark broken on FreeBSD 12+

On FreeBSD 12 and FreeBSD 13, puppetserver6 fails to start. The issue will
likely not be fixed in a timely fashion, so make it clear to end-users that
the port is broken.

As a workaround, users of FreeBSD 12.0 can install puppetserver6 in a
FreeBSD 11.2 jail.

More details, progress and ways to help can be found in the PR 233766.

With hat: puppet
PR: 233766
Reported by: bas@area536.com