docs: Fix deprecation keyword in `.gitlab-ci.yml` by sue445 · Pull Request #4964 · goreleaser/goreleaser

only keyword in .gitlab-ci.yml is now deprecated. c.f. https://docs.gitlab.com/ee/ci/yaml/#only--except So I migrated because GitLab now recommends using rules:if c.f. https://docs.gitlab.com/ee/ci...