More automation in the release notes

[slinkydeveloper]

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.