feat!: Rename branches master->release, nightly->next

[kdmccormick]

TEP: https://discuss.openedx.org/t/tep-rename-nightly-next-and-master-release/12994

GitHub issue: #1096

We also incorporate the IS_NIGHTLY suggestion from this rejected bugfix PR, under the name IS_NEXT, as a reliable way of determining whether to patch for Tutor Release or Tutor Next.

To Do before merging:

• Queue up PRs for the other core plugins
• Make sure Edly is ready to update the GitLib pipelines
• Make sure Regis is ready to rename the branch in GitHub
• Maybe: Wait until after the Sumac release on Dec 9?

Immediately after merging:

• Merge this same commit into Next (the auto-sync may be temporarily broken at this point)
• Rebase this PR and merge it into Next: feat!: Change version suffix from 'nightly' to 'next' #1154
• Merge the core plugin PRs
• Merge: feat: Add "next" index (and symlink "nightly" to it) tpi#6
• Edly: Merge GitLab pipline updates
• Regis: Rename branches in GitHub
• Put an announcement in the forums, x-posted to Slack, explaining how Nightly devs can migrate to Next:

  tutor dev stop
  OLD_ROOT="$(tutor config printroot)"
  git branch -m nightly next
  git switch next
  git pull
  NEW_ROOT=$(tutor config printroot)"
  mv "$OLD_ROOT" "$NEW_ROOT"`
  tutor config save

[kdmccormick]

Suggested change

	For example, if you have ever used :ref:`Tutor Next <next>`, check whether there are still ``tutor_next`` containers running. Conversely, if trying to run Tutor Next now, check whether there are non-Next ``tutor_`` containers running. If so, switch to that other version of Tutor, run ``tutor (dev|local|k8s) stop``, and then switch back to the preferred version of Tutor.
	For example, if you have ever used :ref:`Tutor Next <next>`, check whether there are still ``tutor_next_`` containers running. Conversely, if trying to run Tutor Next now, check whether there are non-Next ``tutor_`` containers running. If so, switch to that other version of Tutor, run ``tutor (dev|local|k8s) stop``, and then switch back to the preferred version of Tutor.

typo

[regisb]

I have an issue with the "IS_NEXT" name, which is not very explicit. Maybe "IS_ON_NEXT_BRANCH"? Or "IS_NEXT_VERSION"?

Also, we can simplify this line to __app__.__version_suffix__ == "next".

[kdmccormick]

@regisb How about just TUTOR_NEXT ? It's not very explicit either, but since "Tutor Next" will be an established term in the docs (like "Tutor Nightly" is now), I think that a user looking at a boolean variable named TUTOR_NEXT will be able to deduce the meaning.