Automate merging releases to staging

[jobara]

Is your feature request related to a problem? Please describe.

Simplify the process of releasing so that more is done in an automated fashion instead of relying on manual tasks via command line.

Describe the solution you'd like

To simplify the release process we should automate merging releases to staging. We had tried to automatically make PRs after a release but at the time it wasn't possible to create PRs from tags (see: #2047). However, we can merge the tag directly into staging when we cut a release and provide an action for merging to production so that the steps don't have to be done manually via command line.

Describe alternatives you've considered

• automatically create release PRs against staging and production (see: Release PRs to staging and production were not generated #2047)
• manually merge the release tag via command line

Additional context

Will need to create GitHub App for the permissions needed to automatically trigger the merging of the release as well as to make the commits that aren't fast-forwards.