Changing gitlab-shell version to 3.6.6 instead of 3.6.3, later on the upgrade pr…

What does this MR do?

This will change the update process to checkout gitlab-shell version 3.6.6 instead of 3.6.3 as currently described in the update document

Why was this MR needed?

To get rid of the message telling you to upgrade gitlab-shell to 3.6.6

Screenshots (if relevant)

!WARNING: This version of GitLab depends on gitlab-shell 3.6.6, but you're running 3.6.3. Please update gitlab-shell."

Does this MR meet the acceptance criteria?

I hope so :)

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

None afaik.