Update CHANGELOG for v1.21.0 release
Update CHANGELOG for v1.21.0 release

healthchecks/healthchecks,iphoting/healthchecks,healthchecks/healthchecks,healthchecks/healthchecks,healthchecks/healthchecks,iphoting/healthchecks,iphoting/healthchecks,iphoting/healthchecks