Closes#320381
Installation with a custom dbtableprefix is not allowed anymore for a
while[1] and we shouldn't advertise it as such.
The option is deprecated for now since I'm not sure if there are some
weird corner-cases where removing the option directly would break
existing installations from before <20 with a custom dbtableprefix. The
migration-path for such a case is as follows:
* Check if /var/lib/nextcloud/config/config.php has the correct
dbtableprefix set and if not, take care of it.
* Remove `dbtableprefix` from the NixOS configuration. It's effectively
state anyways.
After a bit of time to switch (perhaps after the next release
branchoff), the option can be removed.
[1] https://github.com/nextcloud/server/issues/24836
45d2b8b536