Docs: Update API code blocks to use 'plaintext'

What does this MR do?

The code blocks for each API call on these pages did not specify a code style. While 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.

I chose plaintext instead of http for better legibility, because the http lexer renders the API calls in a dull red which I found hard to read. The plaintext lexer renders the text in white, which I think is easier on the eye.

	modified:   doc/api/appearance.md
	modified:   doc/api/audit_events.md
	modified:   doc/api/boards.md
	modified:   doc/api/commits.md
	modified:   doc/api/container_registry.md
	modified:   doc/api/custom_attributes.md
	modified:   doc/api/dependencies.md
	modified:   doc/api/deploy_keys.md
	modified:   doc/api/deployments.md
	modified:   doc/api/discussions.md

Related issues

Related to #32881 (closed)

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

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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.