Use the default value as specified in the yaml

What does this MR do?

It provides a default behavior for experiment feature flag checking to use the default_enabled as specified in the feature flag yaml.

I've not included a changelog because this aspect hasn't been used yet, and so it would likely have minimal impact/has no need to clarify. Happy to add one if there isn't consensus there though.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content