Merge branch 'buster' into 'master'

Buster compatibility

See merge request ai3/thirdparty/dovecot-exporter!1

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