Maintenance: Improve migration testing.

.gitlab/ci/test/migration.yml

@@ -1,20 +1,13 @@
 #
-# Test the migration from the previous `stable` branch to `develop`.
+# Test the migration from the `stable-6.0` branch to `develop`.
 #
 .template_migration:
   stage: test
-  rules:
-    - if: $CI_COMMIT_REF_NAME =~ /^stable/
-      when: never
-    - if: $CI_COMMIT_REF_NAME =~ /^(private|cherry-pick-)/
-      when: manual
-      allow_failure: true
-    - when: on_success
   before_script: []
   script:
     # First, checkout stable and set it up.
     - git fetch --unshallow
-    - git checkout stable
+    - git checkout stable-6.0
     - !reference [.scripts, source_rvm]
     - rvm use 3.1.3
     - !reference [.scripts, bundle_install]