Buster compatibility

Buster ships with Dovecot 2.3, thus switch to "old_stats".

See also https://doc.dovecot.org/configuration_manual/stats/old_statistics/

debian/dovecot-exporter.default

 ADDR=:9166
 SCOPES=global
+DOVECOT_SOCKET=/var/run/dovecot/old-stats

debian/dovecot-exporter.service

@@ -5,7 +5,7 @@ After=dovecot.service
 [Service]
 Type=simple
 EnvironmentFile=/etc/default/dovecot-exporter
-ExecStart=/usr/bin/dovecot-exporter --dovecot.scopes $SCOPES --web.listen-address $ADDR
+ExecStart=/usr/bin/dovecot-exporter --dovecot.scopes $SCOPES --web.listen-address $ADDR --dovecot.socket-path $DOVECOT_SOCKET
 Restart=on-failure
 User=dovecot