Do not enforce using a hash with hidden ci key

What does this MR do?

This MR remove the validation rule that was to strict.

Why was this MR needed?

Enforcing hidden CI job configuration to be a hash, narrows options of using YAML anchors, and was not backwards compatbile.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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?

Closes #21512 (closed)