dovecot: add Flatcurve FTS Engine as EXPERIMENTAL (#5920)

* dovecot: experimental added flatcurve backend + switch

* dovecot: bump docker image

data/Dockerfiles/dovecot/optimize-fts.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [[ "${SKIP_SOLR}" =~ ^([yY][eE][sS]|[yY])+$ && ! "${FLATCURVE_EXPERIMENTAL}" =~ ^([yY][eE][sS]|[yY])+$ ]]; then
+    exit 0
+else
+    doveadm fts optimize -A
+fi