Move merge request approval settings

What does this MR do?

This MR moves the "Merge request approval" settings into their own section.

• CE Port: gitlab-ce!23157
• This is part of #8350 (closed) and #1979 (closed)

Screenshots

Before	After

Promotion still shows under "Merge request" settings:

What are the relevant issue numbers?

• #1979 (closed)
• #8350 (closed)

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.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?