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
euxane