chore: Tune release process

[ka3de]

What this PR does / why we need it:
Modifies the release process in order to be compatible with the usage of release-please and avoid some current manual steps.

Specifically:

• Removes the tag and release generation when promoting build to production, as that should now be done by release-please when merging the version PR.
• Modifies the configuration for the deployment tools PR when promoting to production so it is automatically merged (the same way it's done for staging) instead of requiring a manual approval from reviewers.

Updates https://github.com/grafana/synthetic-monitoring/issues/50.

[ka3de]

To give a little bit more context to other reviewers. This PR is the result of a discussion held with @ckbedwell on how to improve the release process for the app, particularly how to avoid some manual steps, and also how to make it compatible with the usage of release-please (see #933). See the original discussion document.
I applied the changes based on the discussion in that document and my understanding of the app's CI/CD, but I think the simplest approach might be to merge this after #933, try generating a release, and revert It if It does not work 😅