Docs: Update more API code blocks

What does this MR do?

The code blocks for each API call on these pages did not specify a code style. While most of these are HTTP requests (GET, POST, DELETE, PUT, etc.) they aren't structured in the way the http lexer (https://github.com/rouge-ruby/rouge/wiki/List-of-supported-languages-and-lexers) expects. !26016 (merged) has a screenshot and a fuller explanation of why I chose plaintext instead of http.

	modified:   doc/api/merge_request_context_commits.md
	modified:   doc/api/merge_requests.md
	modified:   doc/api/milestones.md
	modified:   doc/api/namespaces.md
	modified:   doc/api/notes.md
	modified:   doc/api/notification_settings.md

Related issues

• Related to !26016 (merged)
• Related to #32881 (closed)
• Related to !26018 (merged)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.