chore: Rename annotate-snippets-rs's default branch to main

[Muscraft]

#t-infra > moving annotate-snippets under t-compiler @ 💬

rust-lang/annotate-snippets-rs#348

[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/annotate-snippets-rs':
      Branch Protections:
          main
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 0
            Requires PR: true
            Required Checks: ["CI"]
            Allowances: []
          master
            Deleting branch protection