[receiver/k8scluster] Dropped support of k8s cluster version earlier than 1.21

[dmitryax]

After #7406 k8scluster receiver works only with 1.21+ versions k8s server API.

If k8s server version is 1.20 or lower, the following error messages are thrown and no metrics are collected:

W0422 20:00:11.629441       1 reflector.go:324] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: failed to list *v1.CronJob: the server could not find the requested resource
E0422 20:00:11.629501       1 reflector.go:138] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: Failed to watch *v1.CronJob: failed to list *v1.CronJob: the server could not find the requested resource
W0422 20:00:13.085439       1 reflector.go:324] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: failed to list *v1.CronJob: the server could not find the requested resource
E0422 20:00:13.085471       1 reflector.go:138] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: Failed to watch *v1.CronJob: failed to list *v1.CronJob: the server could not find the requested resource
W0422 20:00:16.247928       1 reflector.go:324] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: failed to list *v1.CronJob: the server could not find the requested resource
E0422 20:00:16.247961       1 reflector.go:138] k8s.io/client-go@v0.23.3/tools/cache/reflector.go:167: Failed to watch *v1.CronJob: failed to list *v1.CronJob: the server could not find the requested resource

Even if 1.20 is EOL'd already, all the cloud providers like EKS, AKS and GKE still support it.