ci: Remove Bors

[blampe]

Description

This PR removes Bors in favor of native GitHub merge queues. A very similar
cutover was tested and verified in
pulumi/watchutil-rs#39.

Deployment plan

• Find some time where PRs aren't being merged. This is mostly to minimize confusion
  while we're futzing with branch protections.
• Update the repo's branch protection rules to match
  Small PR for testing merge queue watchutil-rs#38:
  • "Allow specified actors to bypass required pull requests": "bors" → Unchecked
  • "Status checks that are required": "bors" → "ci-ok"
  • "Require merge queue": Unchecked → Checked
    • "Only merge non-failing pull requests" → Unchecked
    • "Merge method" → Squash and merge
• Merge this PR using the "Merge when ready" button.
  • If it merges successfully: confirm the prepare-release step ran and published
    pre-release artifacts.
  • If the merge fails: it's possible we'll need to tweak steps slightly
    (e.g. if the step's name doesn't match the branch protection rule). The
    end goal is a "ci-ok" step in both the PR and merge workflows that
    captures whether everything is green.
    • If the issues aren't immediately resolvable, revert the changes made
      to branch protection settings. Folks will continue to merge with
      Bors.

If all goes well, we should be able to continue merging with MQ for a while --
at least long enough to cut a new CLI release. Once we're confident we won't
need to roll back:

• Delete branch protection rules for:
  • staging
  • testing
• Remove pu/pu from https://app.bors.tech/repositories

Fixes #13501

Checklist

• I have run make tidy to update any new dependencies
• I have run make lint to verify my code passes the lint check
  • I have formatted my code using gofumpt

• I have added tests that prove my fix is effective or that my feature works

• I have run make changelog and committed the changelog/pending/<file> documenting my change

• Yes, there are changes in this PR that warrants bumping the Pulumi Cloud API version

[blampe]

Current dependencies on/for this PR:

• master
  • PR ci: Remove Bors #13715 👈
    • PR ci: Remove linear history check #13716

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[pulumi-bot]

Changelog

[uncommitted] (2023-08-12)