docs: fix deprecation keyword in .gitlab-ci.yml (#4964)

`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/jobs/job_rules.html#run-jobs-only-in-specific-pipeline-types

www/docs/ci/gitlab.md

@@ -40,8 +40,8 @@ release:
   image:
     name: goreleaser/goreleaser
     entrypoint: [""]
-  only:
-    - tags
+  rules:
+    - if: $CI_COMMIT_TAG
   variables:
     # Disable shallow cloning so that goreleaser can diff between tags to
     # generate a changelog.
@@ -88,9 +88,8 @@ release:
     GIT_DEPTH: 0
 
   # Only run this release job for tags, not every commit (for example).
-  only:
-    refs:
-      - tags
+  rules:
+    - if: $CI_COMMIT_TAG
 
   script: |
     # GITLAB_TOKEN is needed to create GitLab releases.