Adjust dropdown item and header padding to comply with design specs

What does this MR do?

Change the styling of dropdown items and headers so they comply with the dropdown design specs, namely:

• 12px horizontal padding, 8px vertical padding for dropdown items:

• 12px horizontal padding, 8px vertical padding, 14px font size, text color for dropdown headers:

• 16px line height:

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Before	After

What are the relevant issue numbers?

part of https://gitlab.com/gitlab-org/gitlab-ee/issues/7715

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer