Add warning about JUnit test reports

doc/administration/job_artifacts.md

@@ -160,6 +160,11 @@ _The artifacts are stored by default in
    gitlab-rake gitlab:artifacts:migrate
    ```
 
+CAUTION: **CAUTION:**
+JUnit test report artifact (`junit.xml.gz`) migration
+[is not supported](https://gitlab.com/gitlab-org/gitlab/issues/27698)
+by the `gitlab:artifacts:migrate` script.
+
 **In installations from source:**
 
 _The artifacts are stored by default in
@@ -188,6 +193,11 @@ _The artifacts are stored by default in
    sudo -u git -H bundle exec rake gitlab:artifacts:migrate RAILS_ENV=production
    ```
 
+CAUTION: **CAUTION:**
+JUnit test report artifact (`junit.xml.gz`) migration
+[is not supported](https://gitlab.com/gitlab-org/gitlab/issues/27698)
+by the `gitlab:artifacts:migrate` script.
+
 ### Migrating from object storage to local storage
 
 In order to migrate back to local storage: