v0.7.9

ci: Bump image version in the Harvester repo when tagging

Add a workflow to create a PR in the harvester/harvester repo when seeing a tag event.
The workflow will:
- Determine the stable branch in this repo. e.g., version v0.7.8 -> branch v0.7.x.
- Reading the file `.github/branch-mappings.yaml` and determine the corresponding
  Harvester branch. e.g., v0.7.x -> v1.4
- Clone harvester/harvester repo and modify the component's image tags in
  `deploy/charts/harvester/values.yaml`
- Submit a PR.