Prepare use of GitHub merge queues (#3408)

Merge queues should help us avoid having to merge from main repeatedly
even when all approvals are in and all CI jobs have succeeded. See
https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

Co-authored-by: Felipe R. Monteiro <felisous@amazon.com>

.github/workflows/audit.yml

@@ -7,6 +7,7 @@
 name: Cargo Audit
 on:
  pull_request:
+ merge_group:
  push:
  # Run on changes to branches but not tags.
  branches:

.github/workflows/format-check.yml

@@ -3,6 +3,7 @@
 name: Kani Format Check
 on:
  pull_request:
+ merge_group:
  push:
  # Not just any push, as that includes tags.
  # We don't want to re-trigger this workflow when tagging an existing commit.

.github/workflows/kani.yml

@@ -3,6 +3,7 @@
 name: Kani CI
 on:
  pull_request:
+ merge_group:
  push:
  # Not just any push, as that includes tags.
  # We don't want to re-trigger this workflow when tagging an existing commit.

.github/workflows/release.yml

@@ -8,6 +8,7 @@
 name: Release Bundle
 on:
  pull_request:
+ merge_group:
  push:
  branches:
  - 'main'