Draft: Use whats_new_drawer instead of whats_new_dropdown

What does this MR do?

Consolidates the What's new feature under a single feature flag, rather than two.

TODO: remove unused feature flag YML (and add changelog?)

• What's new is in the ? menu
• when the flag is false, What's new will link to https://about.gitlab.com/releases/gitlab-com/
• when the flag is true, the What's new link will open the What's new drawer
• when the flag is true, the What's new feature can display a notification dot on the ? menu, and a count next to the What's new link (see gitlab-org/growth/engineering#5386 (closed) - currently uses local storage display-whats-new-notification-*

Note https://gitlab.com/gitlab-org/gitlab/-/issues/294184 will ensure code is moved to /ee where applicable

Screenshots (strongly suggested)

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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team