Allow numeric values in gitlab-ci.yml

What does this MR do?

It adds validations to check if a value of a key in gitlab-ci.yml file is a string or a numeric.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30017 (closed)