Alert Users that Feature Flags will be Changing

What does this MR do?

This is to inform users that the feature flag UI will be changing soon. It looks to see if the new UI has been enabled already to decide whether or not to show the alert, as it is a useless alert if the new UI has been enabled.

relevant issue is #217785 (closed)

Screenshots

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done