Update gitlab_flow.md, Teatro seems to be completely dead, see also…

What does this MR do?

It solves a dead link in the documentation

Are there points in the code the reviewer needs to double check?

if it is apropriate to delete the remark

Why was this MR needed?

The documentation was not uptodate and pointed to a non existing service

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• 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?

No issue was created