dovecot_stats_ only works for dovecot 2.1 and 2.2

[Bert Van de Poel]

I got confused after a big migration that dovecot_stats_ was acting up. Turns out, as mentioned on https://doc.dovecot.org/configuration_manual/stats/ , that dovecot stats using doveadm were completely redone in dovecot 2.3 (first release in December 2017, last 2.2 security release in august 2019). The command executed in dovecot_stats_ is therefore no longer valid. I wonder whether the plugin should be marked to only be useable with 2.1 and 2.2, or whether it should be removed altogether.