ci: Use default Ruby version for MRs targeting stable branches

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab-ci.yml

@@ -76,7 +76,7 @@ workflow:
     # they serve no purpose and will run anyway when the changes are merged.
     - if: '$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME =~ /^release-tools\/\d+\.\d+\.\d+-rc\d+$/ && $CI_MERGE_REQUEST_TARGET_BRANCH_NAME =~ /^[\d-]+-stable(-ee)?$/ && $CI_PROJECT_PATH == "gitlab-org/gitlab"'
       when: never
-    - if: '$CI_MERGE_REQUEST_LABELS =~ /pipeline:run-in-ruby3_1/'
+    - if: '$CI_MERGE_REQUEST_LABELS =~ /pipeline:run-in-ruby3_1/ || $CI_MERGE_REQUEST_TARGET_BRANCH_NAME =~ /^[\d-]+-stable(-ee)?$/'
       variables:
         <<: [*default-ruby-variables, *default-merge-request-variables]
         PIPELINE_NAME: 'Ruby $RUBY_VERSION MR'