Change vertical margin of page titles to 16px

What does this MR do?

Make vertical margin of page titles symmetric and aligned to grid.

Screenshots

Before	After

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7433#note_103849055

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