package.mask: Add net-dns/pdns to schedule removal
net-dns/pdns is available in ::gentoo and the older version in the fem-overlay is no longer used by FeM. Thus, we schedule removal after 2022-04-01.
net-dns/pdns is available in ::gentoo and the older version in the fem-overlay is no longer used by FeM. Thus, we schedule removal after 2022-04-01.