Consider removing dependency on AWS ECR and DockerHub registries for release Docker images

To continue the discussion from https://gitlab.com/gitlab-com/runner-group/team-tasks/-/issues/167#note_1012638371

In the cases where we want to remove existing versions from these registries we encountered issues:

• ECR - Seeming inability to remove tags
• DockerHub - 500 internal server errors

@erushton @DarrenEastman in case there is some product reason to have ECR still. For DockerHub I don't think we have any blockers.

/cc @tmaczukin