Release notes for Serving 0.19.0

Changes by Kind

The deprecated monitoring bundle has been removed (#9807, @dprotaso)
Drop serving v1alpha1 and v1beta1 APIs (#9617, #9620, @mattmoor, @dprotaso)
We only mount a volume at /var/log if the operator has enabled log collection
Runtime contract /var/log requirement has changed from MUST to MAY (#9683, @dprotaso)

Adds a Scale Down Delay feature, allowing maintaining replica count for a configurable period after request count drops to avoid cold start penalty. (#9626, @julz)
(Alpha) Adds a DomainMapping CRD in v1alpha1, allowing mapping a custom domain name to a Knative Service (#9714, #9735, #9752, #9796, #9915, #10044, @julz)
Adding cluster-wide flag max-scale-limit. This ensures that both cluster-wide flag max-scale and per-revision annotation "autoscaling.knative.dev/maxScale" for new revision will not exceed this number. (#9577, @taragu)
All of our deployments run with a minimal set of kernel capabilities. (#9973, @markusthoemmes)
Autoscaler now supports multiple pods. Autoscaler Deployment needs to be scaled to 0 first then scaled to other replica value. (#9682, @yanweiguo)
Updated the Service schema to include a high level basic schema. (#9436, #9953, @joshuawilson)
Queue-proxies are no longer allow to run as root, they have a read-only root filesystem and have all capabilities dropped. (#9974, @markusthoemmes)
ResponsiveRevisionGC is enabled by default (#9710, @whaught)
Revisions are now named more clearly and consistently. (#9740, @markusthoemmes)

Domain is validated by k8s library IsFullyQualifiedDomainName(). (#10023, @nak3)
Fixed a rare nil-pointer exception in the autoscaler (#9794, @markusthoemmes)
Ingress is reconciled when label was different from desired. (#9719, @nak3)