Rename the default branch of triagebot to main

[Kobzol]

Related PR: rust-lang/triagebot#2484

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/triagebot':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Require merge queue: true
            Required approvals: 1
            Required status checks: ci (integration_id: 15368)
          Deleting 'master'