Rename default branch of futures-rs and pin-utils to main

[taiki-e]

Per #t-infra > Renaming default branches from master to main, I'd like to rename the default branch name for the repos that I maintain:

• Rename default branch to main futures-rs#3026
• Rename default branch to main pin-utils#40

I don't have permission to change the branch name in the settings, so I would appreciate it if someone from the infra-admin could do that when merging this.

[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/futures-rs':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Required approvals: 0
          Deleting 'master'
    📝 Editing repo 'rust-lang/pin-utils':
      Rulesets:
          Creating 'main'
            Include Branches: ["refs/heads/main"]
            Required approvals: 0