Remove `ci_multi_doc_yaml` FF

What does this MR do and why?

This MR removes the ci_multi_doc_yaml feature flag. Multi-document YAML loading for CI config files has been workingin production for over a milestone, so we're ready to remove the feature flag

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #388836 (closed)