Set GITLAB_HELM_CHART_REF to a commit with stability improvements

What does this MR do?

Since we’re now using stable version of the chart for our Review Apps (!18653 (merged)), that means my improvement (gitlab-org/charts/gitlab!985 (merged)) and @caalberts's improvement (gitlab-org/charts/gitlab!991 (merged)) isn’t used anymore, and we can see that there’s an impact on the readiness errors for unicorn:

This MR changes the GitLab Helm chart version used to deploy the Review Apps so that we can take advantage of our improvements, while still using a fixed version. Once v2.3.8 will be released, we should use it.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content