updates default this-week-in-rust branch name to main

[nellshamrell]

No description provided.

[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] 💻 Team Diffs:
    📝 Editing team 'rust-lang/project-vision-doc-2025':
      Adding member 'ernestkissiedu' with member role
    💻 Repo Diffs:
    📝 Editing repo 'rust-lang/this-week-in-rust':
      Branch Protections:
          main
            Dismiss Stale Reviews: false
            Is admin enforced: true
            Required Approving Review Count: 0
            Requires PR: true
            Required Checks: []
            Allowances: []
          master
            Deleting branch protection