Rename 'master' branch to 'main'

[remkop]

I will rename the 'master' branch of the picocli project to 'main'.

I will do this before releasing the 4.6.3 version.

Contributors will have to manually update their local environments. You may use the following commands to do so:

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