feat: improve version suggestion checks

[pepopowitz]

Originally part of #1778, but extracted so that I could close that issue.

We have a check (check-versions) that makes suggestions about versions being edited in a PR. We could enhance the logic to be a little smarter, including:

• Skip suggestions if a future-version is applied (e.g. target:8.x where 8.x is the next release). In this case, the changes are usually for unreleased features, and don't need to be backported.
• if vPrevious (or older) is affected, but vPrevious+1 through vNext are not, suggest that the changes should also be applied to vPrevious+1 though vNext. It's unusual that we edit an older version of the docs in isolation.