v1.21.0

Changes since v1.20.0

A release process following Kubernetes releases was added to the project.
The major and minor versions of custom-metrics-apiserver are in sync with
upstream Kubernetes and the patch version is reserved for changes in
custom-metrics-apiserver that we want to release without having to wait for the
next version of Kubernetes.

Since the project is in sync with upstream Kubernetes releases, it is following
the same support policies, so alongside this release will be cut two other
releases (v1.19, v1.20) to have the three most recent releases of Kubernetes.

Improvements

• Bump Kubernetes dependencies to v0.21.2 and Golang to v1.16 (@dgrisonnet)

Tooling

• Replace Travis CI by prow (@dgrisonnet)