Automatic merge from submit-queue (batch tested with PRs 46407, 46457)

Remove deletecollection support from namespace object

Namespace storage accidentally picked up deletecollection support from embedding the generic store. If invoked, it skips the custom namespace `Delete()` storage method that enforces finalization, and skips the namespace lifecycle admission plugin that protects immortal namespaces from deletion.

Given the data integrity implications of skipping namespace finalization, I'd backport this as far as we're releasing patch releases.

```release-note
The namespace API object no longer supports the deletecollection operation.
```