Consider adding header styling for GlDropdownDisclosure

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The following discussion from !112195 (merged) should be addressed:

• @ohoral started a discussion: (+3 comments)

  I've updated slightly the styles of the header for the dropdown as I've stumbled across it. Copied the styling from the listbox header. We can drop this commit though

  Before	After

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Implementation plan (draft)

• Add a header slot to GlDropdownDisclosure.
  • It should be wrapped within a div.gl-display-flex.gl-align-items-center.gl-p-4!.gl-min-h-8.gl-border-b-1.gl-border-b-solid.gl-border-b-gray-200.
• Upon integration in GitLab, revisit existing instances that define custom headers:
  • app/assets/javascripts/batch_comments/components/preview_dropdown.vue.