Fix links to instance-level approval settings

What does this MR do?

In https://gitlab.slack.com/archives/C01EMBKS5DW/p1704463527673199, @leetickett-gitlab reported he was having trouble finding the instance-level config for MR approvals settings. He (correctly!) pointed out that links purporting to go to instance-level instructions were pointing to https://docs.gitlab.com/ee/administration/admin_area.html, which isn't the right page.

• Wrong: https://docs.gitlab.com/ee/administration/admin_area.html
• Correct: https://docs.gitlab.com/ee/administration/merge_requests_approvals.html

It tickled a memory, though. A few months ago, we moved a lot of pages in doc/user/admin_area/ to a new place in the doc/administration hierarchy. Was this related?

I found the instance-related lines in doc/user/project/merge_requests/approvals/settings.md and used git blame to walk backward to see what those lines previously linked to. Answer: doc/administration/merge_requests_approvals.md.

In !138223 (merged) the redirect for doc/user/admin_area/merge_requests_approvals.md → doc/administration/merge_requests_approvals.md was removed, and links to doc/administration/merge_requests_approvals.md were updated to the wrong place. It was pretty easy to reconstruct what had happened - a search + replace for merge_requests_approvals.md changed more links than it should've.

This MR corrects those links that were changed.

Related issues

• Related to Clean up docs redirects, ee - 2023-11-29 (!138223 - merged) where the redirects were removed, and URLs replaced
• Related to Clean up docs redirects - 2023-11-29 (gitlab-docs!4435 - merged) which is when ^^ was done.

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