Make use of the new `release` CI keyword in ci/gem-release.yml

The release CI keyword was recently released (https://docs.gitlab.com/ee/ci/yaml/#release), and it would be good to dogfood it! We already have https://gitlab.com/gitlab-org/quality/pipeline-common/-/blob/master/ci/gem-release.yml where we manually call the API to create the tag but we don't create the release yet.