Change branch protection of rustfmt

[Kobzol]

Requested in https://rust-lang.zulipchat.com/#narrow/channel/357797-t-rustfmt/topic/Rename.20primary.20branch.20to.20.60main.60/with/553532758.

[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/rustfmt':
      Branch Protections:
          main
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 0
            Requires PR: true
            Required Checks: ["(i686-pc-windows-gnu, nightly)", "(i686-pc-windows-gnu, stable)", "(i686-pc-windows-msvc, nightly)", "(i686-pc-windows-msvc, stable)", "(x86_64-apple-darwin, nightly)", "(x86_64-apple-darwin, stable)", "(x86_64-pc-windows-gnu, nightly)", "(x86_64-pc-windows-gnu, stable)", "(x86_64-pc-windows-msvc, nightly)", "(x86_64-pc-windows-msvc, stable)", "(x86_64-unknown-linux-gnu, nightly)", "(x86_64-unknown-linux-gnu, stable)", "rustdoc check"]
            Allowances: []
          master
            Deleting branch protection