mdBook: update configuration to target main branch

[DanielEScherzer]

No description provided.

[DanielEScherzer]

Cross link rust-lang/mdBook#3206

[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/mdBook':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Merge queue wait time for min group size: 1
            Require merge queue: true
            Required approvals: 1
            Required status checks: Success gate (integration_id: 15368)
          Deleting 'master'
      Environments:
        🔄 Update: publish
            Adding branches: main
            Removing branches: master

[ubiratansoares]

Hey @DanielEScherzer, I can help with this renaming