Drop `Alpine` versions `3.12` , `3.13`, and `3.14` from list of base images for which we build Runner images

Based on the outcome of https://gitlab.com/gitlab-org/gitlab-runner/-/work_items/29619?iid_path=true, we want to drop versions 3.12 , 3.13, and 3.14 of alpine form the base images from which we create runner release images. Those two versions are EOL and don't receive security updates.