Add policy to handle unmaintained dependencies

What does this MR do and why?

When we stumbled upon an unmaintained package in our codebase that was widely used it was unclear how to handle this situation. I think it's sensible to add a policy on how to handle this case that allows for continued flexibility for less used packages but adds a how-to guide for widely used packages.

This MR was originally a Handbook MR: gitlab-com/content-sites/handbook!10295 (closed) and has received some feedback there. Please also refer to that MR for the full feedback history.

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