Adjust divider margin to comply with design specs

What does this MR do?

Change the styling of dividers so they comply with the dropdown design specs, namely:

• 4px vertical margin:

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Before	After

What are the relevant issue numbers?

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

References

• https://github.com/twbs/bootstrap/blob/v4.1.3/scss/_dropdown.scss#L103-L106
• https://github.com/twbs/bootstrap/blob/v4.1.3/scss/mixins/_nav-divider.scss#L5-L10

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