Remind author about a stale Backport PR

[gatherisum]

This issue will be used to document the discussion about the way to remind people about stale Backport PRs.

Idea

Sometimes we forget to merge backports. When a backport becomes stale it means we miss out on valuable bug fixes that we’ve already build for other versions.
Let’s see if we can improve our backport workflows. Some ideas are:

• Auto-request review from original author
• Improve the Backport Dashboard issue
• Add opened backports to a GH project
• Add a slack notification

Goals

• Reduce the number of stale backports
• Make it easier for authors to take ownership of their backports (help them be a good manager of one)
• Build insights into our pending backports across all repositories