Add changelog entry for new dropdown styles

What does this MR do?

Add the missing changelog entry for the changed dropdown styles.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28921

changed milestone to %10.0

@annabeldunstone could you merge this changelog entry for the dropdown changes?

assigned to @annabeldunstone

Yup! Thanks @winh

approved this merge request

merged

mentioned in commit b7c8c5c8