Nextcloud 31 compatible release

[juliusknorr]

Since there was some breaking changes in the storage layers #1522 we cannot force enable collectives, so it would be good to have a 31 compatible release soon.

We can also do a beta/pre release but I think @mejo- just used to do a minor bump in the past.

[max-nextcloud]

@juliusknorr #1522 has already been included in 2.15.2 which was released 3 weeks ago, right?
It looks like it was even included in 2.15.1 and 2.15.0 already.

So it should be possible to force enable collectives.

[juliusknorr]

Right, sorry I just looked at https://apps.nextcloud.com/apps/collectives not the actual change.

Then we can skip the pre-release and just need to bump for the final with 31 as a max-version