Support Nextcloud 31

Needs https://github.com/nextcloud/neon/issues/2720. Current upstream bugs: - [x] https://github.com/nextcloud/server/pull/50425 - [x] https://github.com/nextcloud/spreed/pull/14209 - [x] https://github.com/nextcloud/tables/pull/1557 Updating the specs was relatively smooth, although I hit some minor problems. Testing against the latest git versions of server and all apps went fine except for the issues listed above.