compose: fix dovecot tagging

docker-compose.yml

@@ -229,7 +229,7 @@ services:
             - sogo
 
     dovecot-mailcow:
-      image: mailcow/dovecot:2.21
+      image: mailcow/dovecot:2.3
       depends_on:
         - mysql-mailcow
         - netfilter-mailcow