Render stages in valid order

What does this MR do?

Currently stages in stages dropdown on builds page are not in valid order. It is important to display stages in other those are defined in the pipeline.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21133

Added ~149423 label

Marked the task CHANGELOG entry added as completed

Thanks @ayufan, LGTM! ❤️

Enabled an automatic merge when the build for 2baa90b2 succeeds

Mentioned in issue #21277 (closed)

Status changed to merged

Mentioned in commit f30005f0

Picked into 8-12-stable, will go into RC6.

Removed ~149423 label

Mentioned in commit a4e24cd6