Migrate from master to main

[HonkingGoose]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Adjust workflow files
• Adjust links in some files on our repo (not strictly necessary, but nice to have anyways)
• Adjust atlas.json
• Adjust line in TRANSLATING.md that refers to the ProGit2 default branch

Context

Well waiting on this has been very beneficial, as GitHub does most of the hard work automatically now. 😄

Read the GitHub docs before starting: https://docs.github.com/en/github/administering-a-repository/managing-branches-in-your-repository/renaming-a-branch

Steps to migrate:

1. Review this PR, confirm this contains all the necessary changes in script files/rakefile/action workflow files.
2. Merge this PR.
3. Repository maintainer uses GitHub web interface to rename the master branch to main following the steps in the GitHub docs.
4. Done.

State of repository after merging this and renaming master -> main:

• Anybody who has forked progit2 will get a message from GitHub telling them what they need to do to update their forks.
• Anybody who links to our current master branch will get redirected to the main branch automatically, with no breaking links/scripts on their end.
• GitHub will make sure all open PRs are targeting the main branch
• Dependabot will update its PRs to target main as well

Fixes #1463.

[ben]

Done! Please rename your local master branches to main. Thanks!

[HonkingGoose]

Hooray! 😄 ✨