Release Environments - Integrate GET-based environment to deployment pipeline

Context

One Exit Criteria of &1567 (closed) is to deploy Release Environments (RE) using GitLab Environment Toolkit (GET), so RE can support GitLab Geo setup.

In #21234 (closed) and #21235 (closed), a fully automated GET setup was prepared ./get ). However, it was not linked to the current RE deployment workflow, so the deployment doesn't happen automatically yet.

This issue aims at triggering a new deployment pipeline to deploy a GET environment whenever a new commit is added to a stable pipeline, similar to what is currently done with the Helm setup. Since the GET environment is still on the testing phase, it can have allow_failure: true and "trigger and forget", so it doesn't block and waste waiting time.

Exit Criteria

• The latest version of a stable branch is deployed to a GET-based RE whenever a new commit is added to the branch.
• Create an issue for deploying to all GET env all at once (#21334)