featureflag: Enable atomic repository creation by default

With f87bc1e9 (Merge branch 'pks-create-repository-atomic' into 'master', 2021-11-24), we have implemented support for atomic repository creation. With this mode, creation of repositories will use locking and meaningful transactional voting to create repos in a race-free manner.

The feature flag guarding the new behaviour has been rolled out on December 6th, and no issues have surfaced since then. But the change is rather high-risk given that it changes semantics of some of the RPCs to return errors in case the target repository exists already, which wasn't the case previously. To be on the safe side, we thus want to keep this feature flag for at least one release such that we have an escape hatch in case there are issues with this change which we haven't yet seen.

Default-enable the flag.

Changelog: changed

Part of #3941 (closed)