Use release notes from the PR content

[naveensrinivasan]

Describe the bug
At the moment, we use PR titles release notes. This isn't helpful for someone who wants to know what has changed ossf/scorecard-action#786

Reproduction steps
We adopted a section in the PR to help with release note generation. We should utilize that.

[naveensrinivasan]

@justaugustus 👀

[spencerschrock]

This was added to the release instructions in #3322