Change the default branch name from master to main

Related: https://gitlab.com/gitlab-org/gitlab/-/issues/221014

Current status: done.

• Create main branch from master
• Update default branch to main in repository settings and set protected branch
• Update all open merge requests to point to main
• Update .gitlab-ci.yml and documentation !437 (merged)
• Update all downstream users of master
  • Omnibus omnibus-gitlab!4772 (merged)
  • CNG gitlab-org/charts/gitlab!1670 (merged) <- gitlab-org/build/CNG!558 (merged)
  • GCK gitlab-compose-kit!143 (merged)
  • [-] GDK
  • GitLab docs (gitlab!48343 (merged))
  • Security group mirror's default branch + protected branch settings
  • Dev instance mirror's default branch + protected branch settings
  • GitHub instance's default branch
  • CodeClimate
• Delete master so we no longer have to keep it up to date