Move latest ReleaseNotes.md entries to CHANGELOG.md and remove ReleaseNotes.md

Now that we have a new version tagged, we are moving away from the manual updating ReleaseNotes.md and relying on auto-generating entries in CHANGELOG.md based on commits instead. As such, we should make sure that CHANGELOG.md has all the changes from the latest tag and then we can remove ReleaseNotes.md.