Explore GitHub options to automatically update develop onto approved automerge and tested PR branch

[williamfgc]

Is your feature request related to a problem? Please describe. Currently, when PRs come simultaneously they get out-of-sync with any changes to the target develop branch. The goal is to update automatically an already approved and tested PR branch (currently merging or rebasing develop).

Describe the solution you'd like Tested and approved PRs shouldn't go out of sync, and ideally be rebased from the latest develop branch.

Describe alternatives you've considered Options to explore: GitHub Actions that could do these (if any?) or a GitHub probot

Additional context This could be a bit invasive to the PR owner/contributors as it will make any pre-existing local branch non fast forward with the remote PR branch.

[prckent]

The content of the title is important: I think we only want this for PRs that have been approved for Automerge and where the tests have been run successfully once. Therefore the admins will be able to approve for automerge a handful of PRs and they will be updated and merged in sequence. Currently this requires human steps for every PR.

[ye-luo]

I'm not sure if we'd like to do rebase. merge develop into the feature branch is more common.

[walshmm]

#3878 is ready to be looked at, but I will leave it in a draft state as I'd want to clean up commits before it got merged.