Rename the master branch to main

[zufuliu]

I was gonna ask to rename the master branch to main at some point, it's a good decision.

Originally posted by @Shakil-Shahadat in #346 (reply in thread)

[zufuliu]

As documented at https://github.com/github/renaming, this can be done now with a single mouse click, thanks for their hard work👍.

I'm planing to rename the branch name in this week, so our first commit (unless there are regressions for v4.21.09, workaround for issue #378 can be done in this week) for v4.21.11 goes into the main branch.

[zufuliu]

Note to my self (for steps to update local repository):

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

git remote show origin
git remote prune origin

[zufuliu]

Finished after 574cb1e.