dovecot/fts: switch to fts-flatcurve

This switches the full-text search plugin from fts-xapian to
fts-flatcurve, the now preferred indexer still powered by Xapian,
which will be integrated into Dovecot core 2.4.

This sets a sane minimal configuration for the plugin with
international language support.

The plugin options marked as "advanced" in Dovecot's documentation
aren't re-exposed for simplicity. They can nevertheless be overridden
by module consumers by directly setting keys with
`services.dovecot2.pluginSettings.fts_*`.

The `fullTextSearch.maintenance` option is removed as the index is now
incrementally optimised in the background.

GitLab: closes https://gitlab.com/simple-nixos-mailserver/nixos-mailserver/-/issues/239

docs/fts.rst

@@ -4,7 +4,7 @@ Full text search
 By default, when your IMAP client searches for an email containing some
 text in its *body*, dovecot will read all your email sequentially. This
 is very slow and IO intensive. To speed body searches up, it is possible to
-*index* emails with a plugin to dovecot, ``fts_xapian``.
+*index* emails with a plugin to dovecot, ``fts_flatcurve``.
 
 Enabling full text search
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -59,7 +59,8 @@ Mitigating resources requirements
 
 You can:
 
-* increase the minimum search term size ``mailserver.fullTextSearch.minSize``
+* exclude some headers from indexation with ``mailserver.fullTextSearch.headerExcludes``
+* disable expensive token normalisation in ``mailserver.fullTextSearch.filters``
 * disable automatic indexation for some folders with
   ``mailserver.fullTextSearch.autoIndexExclude``.  Folders can be specified by
   name (``"Trash"``), by special use (``"\\Junk"``) or with a wildcard.