ui: Implement `ParentElement` for `Banner` (#29834)

This PR implements the `ParentElement` trait for the `Banner` component
so that it can use the real children APIs instead of a bespoke one.

Release Notes:

- N/A
7f8e3fd482Marshall Bowers committed on 5/3/2025, 2:36:53 AM· committed by GitHubparentf0515d1
3 files changedLine totals unavailable