thanks: update github-pages environment to target main branch

[DanielEScherzer]

No description provided.

[DanielEScherzer]

Cross linking:

• Prepare to rename rust-lang/thanks primary branch to main promote-release#108
• Prepare to rename primary branch to main thanks#100

[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/thanks':
      Environments:
        🔄 Update: github-pages
            Adding branches: main
            Removing branches: master

[Kobzol]

Could you rebase this, please? Thanks!

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.