Run CI migrations since 11.11.0

One can not upgrade straigt from 9.3 to 12.0, due to the schema changes
being backported. Instead, one must first upgrade to 11.11.X. Omnibus
already enforces this, and our online upgrade guidelines also cover
this.

By changing the CI config we don't have to adjust the backport migration
to handle a migration path that one can not take.

.gitlab/ci/rails.gitlab-ci.yml

@@ -111,7 +111,7 @@
   variables:
     SETUP_DB: "false"
   script:
-    - git fetch https://gitlab.com/gitlab-org/gitlab-ce.git v9.3.0
+    - git fetch https://gitlab.com/gitlab-org/gitlab-ce.git v11.11.0
     - git checkout -f FETCH_HEAD
     - sed -i "s/gem 'oj', '~> 2.17.4'//" Gemfile
     - sed -i "s/gem 'bootsnap', '~> 1.0.0'/gem 'bootsnap'/" Gemfile