Change release pipeline to promote dev manifests

Up until now, we've been releasing new versions of the manifest by bumping the old manifest's semver contained in the file name to the new target version, e.g., `0.1.35.yaml` became `0.1.36.yaml`. This approach isn't great because it does not allow us to make content changes without modifying a release set of manifests.

Change our release process (and, correspondingly, the Github Actions) to promote the `dev.yaml` file we introduced in #412. We have done something very similar in csi-digitalocean, so we can probably steal most of the logic from there.