docs: clarify expectations for unhandled dependency update PRs

[BethGriggs]

As discussed in the Community Plugins SIG.

While working on this, I realised it might be more effective to tie this process to the existing stale bot. That way, instead of manually tracking timeline, the @backstage/community-plugins-maintainers team can filter by the stale and dependencies labels to identify which PRs need action. It also means that if we tweak the staleness timeframes in the bot configuration, this process would automatically stay aligned with those values.

✔️ Checklist

• All your commits have a Signed-off-by line in the message. (more info)

[awanlin]

Thanks @BethGriggs, this makes a ton of sense to me 👍

[Parsifal-M]

I like this because it's always been a pain point for me having to decide whether to merge something or not that is a dependency across multiple workspaces (even if a patch/minor)