Migrate away from bors, use a Github merge queue instead.

[Bors is deprecated and the public instance will be shutting down.](https://bors.tech/newsletter/2023/05/01/tmib-76) We should migrate to a [Github merge queue](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue) instead.