2972: switch to fts-flatcurve r=mergify[bot] a=nextgens
## What type of PR?
bug-fix
## What does this PR do?
Switch from fts-xapian to fts-flatcurve. This should address the problem with indexes getting too big and will be the default in dovecot 2.4.
Don't forget to nuke old indexes to reclaim space.
### Related issue(s)
- close#2390
- close#2184
- close#2976
## Prerequisites
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [ ] In case of feature or enhancement: documentation updated accordingly
- [x] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/workflow.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
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