Run Yamllint for docs deprecations removals

What does this MR do and why?

Related to #354072 (closed)

We should ensure our deprecation, removal, and what's new YAML files all follow standards. This MR adds them to the list of files that we check when we run yamllint. The vast majority of issues were already fixed in separate MRs as part of the related issue, and the final 3 are fixed in this MR to demonstrate that the test is working.

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