More automation in the release notes

From https://groups.google.com/g/knative-dev/c/jQsloZD-sW8/m/8E31LeFGCgAJ

> Need more and better automation for the release notes. Or we completely automate them (and fix later), so when prow creates the release runs the release-notes script and applies that to the github release, or we need a better way to collaborate on them for every repo (although some repos like sources doesn't really need them, some "bigger" downstream repos like kafka needs them). I'll open an issue for that.

Start by reviewing how prow creates releases and how the release-notes script currently works. Define done as either applying generated release notes to the GitHub release automatically or establishing a better collaboration workflow for repositories with different release-note needs.

Written by the indexing model from the issue text.