Prepare to rename the default branch of amp-github-apps to main

[rsimha]

PR Highlights:

• Sets the stage to change the default branch of amp-github-apps to main
• The only instances of master that will remain in this repo are links to meta, amphtml-build-artifacts, amphtml, and error-tracker. They'll need to be updated separately.

Note: Looks like prettier auto-formatting kicked in for the .md files I updated. I'm going to leave those changes in.

Next steps: Soon after this PR is merged, we can rename the default branch to main. After review, I'll coordinate this action manually. This will serve as a practice run for the more involved switch for amphtml 😃

Partial fix for ampproject/amphtml#32195
Accompanying PR to ampproject/amphtml#33571

[rcebulko]

Right now, there is no main branch for this repo, so workflows could fail and links will be dead in the interim. Have we considered creating a main branch and keeping them automatically sync'd (this could be as easy as a cloud function running every minute that makes main point to HEAD of master)? This would be a lightweight solution, easily applicable here and to all the other repos, that would give plenty of time for all the pieces to migrate safely

Edit: This would also provide a wider window for developers to adapt their workflows/IDE/git settings to the new branch.

[rsimha]

Right now, there is no main branch for this repo, so workflows could fail and links will be dead in the interim.

According to the GitHub documentation, links will auto-redirect, draft PRs will be auto-transitioned, etc.

Have we considered creating a main branch and keeping them automatically sync'd (this could be as easy as a cloud function running every minute that makes main point to HEAD of master)? This would be a lightweight solution, easily applicable here and to all the other repos, that would give plenty of time for all the pieces to migrate safely

This is an ongoing topic of discussion. I've written up a doc with a high-level plan and listed this as an alternative. Based on current evidence and GitHub's built-in features, I think renaming the branch is preferable to creating a duplicate and syncing commits, but I am open to looking at more evidence in favor of / against this plan. Let's continue the discussion via doc comments before taking any further action on this PR.

[rsimha]

Bumping this PR now that a transition plan has been published.

Once this PR is reviewed and merged, I will rename the branch, immediately merge this PR, and make sure that all post-merge CI ran fine against the correct branch.