Rename default branch of rustup to main

[Kobzol]

Related PR: rust-lang/rustup#4511

[github-actions]

Dry-run check results

[WARN  sync_team] sync-team is running in dry mode, no changes will be applied.
[INFO  sync_team] synchronizing github
[INFO  sync_team] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/rustup':
      Branch Protections:
          main
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 1
            Requires PR: true
            Required Checks: ["conclusion"]
            Allowances: []
          master
            Deleting branch protection