Phlex repository group-merge readiness. #1014

Merge queues are an interesting optimization of CI use and increase assurance the branch is never broken by incompatible changes.

The Celeritas project enabled this feature in the PR celeritas-project/celeritas#2528.

Depending on how the push event CI step is implemented this may and may not require CI changes, it may also require change in the repository branch ruleset.

A few observation from the Celeritas case which may or may not apply to Phlex:

• PR checks still run the pull_request action which must pass before it's added to the merge queue. (This action has pre-checks etc. to avoid expensive failures, and as usual I think it merges the latest upstream develop.)
• The merge button at the end of a PR says "Merge when ready" rather than "auto-merge" or "merge"; however, it always says this, even if there's a conflict or if the tests haven't run.
• After merge-when-ready and the basic requirements pass, a separate merge_group action is started that merges the head of the merge queue as a conditional develop.
• After each queued MR passes, it's merged by @github-merge-queue rather than whoever hit the merge button. (Also the commit date is set to GMT.)