cleanup cronjob deleted nightly releases needed to serve watershed update

[bhearsum]

In https://bugzilla.mozilla.org/show_bug.cgi?id=1893487 we noticed that a recently created partials-only watershed stopped working. This was because the releases that are needed to match client requests to the partials got cleaned up, because they themselves are not pointed at by a rule.

We should be keeping releases that are needed to make partials work.

I think what we should do is not delete releases that are referenced by a release being kept for another reason. That is to say: we should keep releases that are referenced directly by a releases that is referenced by a rule, but we should not do this recursively (otherwise we'd never delete old nightly releases at all).