[Breaking Change] Report should link to prior preview with approved breaking changes

Breaking changes approved in a preview version will be flagged for review in all subsequent api-versions until they finally make it to a GA version. These redundant reviews are wasteful, consuming reviewer time and slowing down service teams.

In an ideal world such "re-appovals" would be automated, but that would require checking that no new breaking changes were introduced in the new api-version. However, there is part of the process that could be automated -- checking for a prior preview, finding the relevant PR, and checking to see if it has approved breaking changes.

Specifically, I would like the Breaking Changes report to add a message near the top of the report -- only added when there is a prior preview with approved breaking changes -- that says something like:

> Breaking changes may have been previously approved in [PR link].