Mention backporting of pipeline fixes in maintenance policy doc

What does this MR do and why?

Mention backporting of pipeline fixes in maintenance policy doc

Backporting of fixes that help keep stable branch pipelines successful is important because it prevents future security releases from being blocked by failing pipelines.

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Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.