Create the next release branch for dependencies

In order to receive automatically created reviews for dependency updates for
the next release, we need to create the c9s-<next release>-rdo branch.

This needs to be done after the creation of the branch stable/<release> in openstack/requirements

To do so, we have to cut the current release branch to create c9s-<next release>-rdo one.
e.g: https://review.rdoproject.org/r/c/config/+/45467