Tracking issue for Homu (old Bors) deprecation

[Kobzol]

Homu is an implementation of the Bors merge queue bot. It's all kinds of cursed, so we want to get rid of it, and use either native GitHub merge queues (where possible) or the new bors implementation in the rest of the repos (mainly https://github.com/rust-lang/rust).

Getting rid of homu could allow us to also get rid of the rust-lang-ci organization and just run all CI under the rust-lang org and it would also help with getting rust-lang/rust managed under team (rust-lang/team#1463).

Below is a list of things that we need to resolve to reach that goal (in more or less the required order):

• Reduce the amount of repositories that use homu, even though they do not have to.
  • Reduce the amount of repositories that use homu, even though they do not have to #169
• (optional) Make it possible to configure GitHub merge queues in team. Sadly, it is not possible to do using the current API, because GitHub refuses to support it. We would probably need to switch from branch protections to merge rules. This is not a big issue though, only a few repositories use merge queues, and infra-admins can configure these manually, as it's a one-time action (more or less, see the linked document below).
  • https://hackmd.io/@ehuss/HyMezs5n2#GitHub-API-does-not-support-configuring-merge-queues
• Make it possible to configure homu/bors per branch protection, not just per repository. This would allos us to specify more precisely which branches are managed by homu, and also to use both homu and the new bors on the same repository.
  • Add merge-bots configuration per branch protection team#1576
• Implement support for the new bors into team and sync-team. Allow specifying e.g. bors2 in the bots array, and implement installation of the bors GitHub app in sync-team.
• Configure CI permissions/secrets for the new bors. To avoid Partial credentials found in env, missing: AWS_SECRET_ACCESS_KEY.
  • Set up CI in rust-lang/rust for new bors #125
• Use the new bors for try builds on rust-lang/rust
  • Either redirect @bors try to new bors, or add a hint to homu that people can do @bors2 try instead.
• Implement merge functionality in the new bors.
• Swap current repositories in team to use bors instead of homu
• Swap CI to run on rust-lang instead of rust-lang-ci
• Remove homu