Enable the kubernetes agent internal api by default

What does this MR do?

Enable the kubernetes agent internal api by default (:kubernetes_agent_internal_api flag)

Since we now have the JWT internal auth since bf5b5cf9, we don't need the protection of FF anymore. However we still keep the FF so that we can disable this if necessary (e.g. for performance reasons).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required), forthcoming in #235633 (closed)
• 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. https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11237