Change master branch to main.

[mvanaken]

We have been talking about it for so long. Let's do it!

This is mostly a settings change and itself does not require code-changes, but we do have links in our README that has the default branch name in the url.

[mvanaken]

I followed these steps (https://www.git-tower.com/learn/git/faq/git-rename-master-to-main):

• git pull
• git checkout master
• git branch -m master main
• git push -u origin main
• git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
  And then changed the default branch in Settings > General to switch to main branch.

Everybody else can just do locally:

• git pull
• git checkout main
• git branch -d master

The master branch still exists, but is marked read only to avoid accidental pushes to this branch.

I will now open a branch to make small url changes.