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

[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 TUTOR_BRANCH_IS_MAIN, as a reliable way of determining whether to patch for Tutor Release or Tutor Main.

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 Main (the auto-sync may be temporarily broken at this point)
• Rebase this PR and merge it into Main: Change version suffix to 'main' and load plugins from their main branches #1154
• Merge the core plugin PRs
• Merge: Add Main index (and symlink Nightly to it) & Point at main plugin branches 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 Main:

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

[kdmccormick]

@regisb @DawoudSheraz I believe that this, #1154, and overhangio/tpi#6 are all ready to go. Of course, the exact sequencing of them and will depend on how you administer the branch renaming on the overhangio/Edly side.

I'm on holiday from now until Monday, so feel free to push changes to this PR, squash, rebase, merge, whatever you need to.

[regisb]

Since Kyle is OOO, I have taken the liberty to add some changes to the PR. Let me know what you think @DawoudSheraz and then we can proceed with the branch renaming.