Add SSL cert mount to registry init container

What does this MR do?

Add gitlab.certificates.volumeMount to volumeMounts for the dependencies init container.

Currently pods fail to start when SSL is enforced on the database connection (e.g. using sslmode: verify-full) because the dependencies init container can't read the necessary SSL certificate.

Since this is fixing existing functionality I don't think any documentation or test updates (other than checking that the chart renders properly etc.) are needed, and as far as I can tell omnibus-gitlab isn't affected either.

Related issues

Somewhat related to #2034 (closed)

Author checklist

For general guidance, please follow our Contributing guide.

Required

For anything in this list which will not be completed, please provide a reason in the MR discussion.

• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated.
• Have you validated that your change works end-to-end on a real cluster deployment?
  • Describe how you did it in a comment.
• Equivalent MR/issue for omnibus-gitlab opened.
• When ready for review, MR is marked as ready (not a draft) and assigned to a reviewer.

Reviewers checklist

• As soon as you begin reviewing, select Start a review on your first comment so the MR shows Reviewer started review.
• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.