Add bors merge bot

[Kobzol]

This is mostly done to document which branches of rust-lang/rust are managed by bors. The actual bors configuration will have to be configured manually by infra admins (as a reminder, we don't handle GitHub app authorization in team, because it wasn't compatible with the way we setup tokens for the dry-run).

After this is merged, we have to update repositories who access the repos.json endpoint, so that they will be forward-compatible with the bors option, which I will add to rust-lang/rust in a future PR. I think that should be only triagebot.

[rustbot]

rust_team_data/src/v1.rs has been modified, it is used (as a git dependency) by multiple sub-projects like triagebot, the www.rust-lang.org website and others.

If you are changing the data structures, please make sure that the changes are not going to break serde deserialization (adding a field is fine; removing or renaming a field isn't).

If you must do a breaking change to the format, make sure to coordinate it with all the users of the rust_team_data crate.

[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 crates-io
[INFO  sync_team] synchronizing github
[INFO  sync_team] 💻 Repo Diffs:
    📝 Editing repo 'rust-lang/bors':
      Rulesets:
          Branch protection for main

[marcoieni]

we have to update repositories who access the repos.json endpoint

how? can you prepare a PR?

Also, should I merge this PR now?

[Kobzol]

how? can you prepare a PR?

I can't, because the PR is essentially running cargo update on triagebot, and that has to be done after this PR is merged :)

It should be safe to merge this PR, because no repo actually uses bors yet. So:

1. Merge this PR
2. Update triagebot
3. Merge Configure bors for rust-lang/rust #2203