ci: Add backport workflow

[lann]

This will allow merged PRs to be automatically backported to a release
branch via either a label (before the original PR is merged) or a
comment.

Uses:
https://github.com/marketplace/actions/backport-merged-pull-requests-to-selected-branches

[itowlson]

Does the slash command need to say what branch to backport onto, or does it Just Know? Never mind, saw your command about the branch name in Slack

[lann]

Hmm...for some reason I thought you would specify /backport <branch> but after a closer look I think you actually have to add the label and then trigger it with the comment (if you already merged).

[itowlson]

I guess we need another action to add the label then. </deadpan>

[michelleN]

Looks awesome. Will wanna add some guidance to the release process around this once we've tested it out.

Also, we might want to consider that the /backport comment might be abused where as we can set permissions for who can add labels.

[lann]

/backport

[github-actions]

Successfully created backport PR for v1.0:

• [Backport v1.0] ci: Add backport workflow  #1276

[lann]

Also, we might want to consider that the /backport comment might be abused where as we can set permissions for who can add labels.

I don't think the action does anything that a user couldn't do manually, so we should be OK.