Change the default branch from `master` to `main`

In order to support the discussion and our initiative of changing the default branch name, we should follow this GitLab Issue

Let's rename master to main

• Make sure any branch name depending scripts are updated
• Make sure there are no blockers to make this change
• Change the contribution guidelines to reflect the non-standard default branch name
• Set main as default branch
• change target branch of all open merge requests to main
• unprotect master, delete it, and protect master branch again, allowing no one to create it
• propagate the changes to the security repo

A similar project that just went through this change: gitlab-svgs#127 (comment 378289361)