[Updater] When a Dependency Group is refreshed as part of a scheduled GroupUpdateAllVersions, it defers rebasing

[brrygrdn]

Fix for #7310

When testing #7310 yesterday we realised that any attempt to 'rebase', i.e. call the update_pull_request endpoint, from within a scheduled GroupUpdateAllVersions job fails as the endpoint expects the job to specify a single target PullRequest to update.

After discussing this, we realised that having rebases happen within the scheduled job is something we can and should defer - rebases should honour the user configuration and only be triggered when the branch goes into conflict, it's not intentional nor well understood that we rebase during a schedule for some PRs right now.

This change course-corrects our approach so that the delegation to the RefreshGroupUpdatePullRequest will defer rebases but will replace or supersede PRs if the dependencies or versions involved have changed respectively.