Update k8s dependencies to v1.21.0

[sibucan]

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This PR upgrades all Kubernetes-related dependencies from v1.19.2 to v1.21.0. This includes some changes on how the CCM command is initialized, which is explained in https://github.com/kubernetes/kubernetes/blob/v1.21.0/cmd/cloud-controller-manager/main.go and https://github.com/kubernetes/cloud-provider/blob/master/sample/README.md

Now that the Linode CCM is using v1.21.0 k8s dependencies, it should fix #107

[thorn3r]

tested and working 👍

[0xch4z]

LGTM