Make `upcoming-v4` the main branch.

[essiembre]

• Rename upcoming-v4 to main.
• Make main the default git branch.
• Rename master to branch-3.x.

[essiembre]

To update your local branches to the new names:

To rename upcoming-v4 to main:

git branch -m upcoming-v4 main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a

To rename master to 3.x-branch:

git branch -m master 3.x-branch
git fetch origin
git branch -u origin/3.x-branch 3.x-branch
git remote set-head origin -a