Skip the verification of the kubelet's certificate

This is currently needed only for Gardener managed seeds, because
the Kubelet's certificate is not issued by the default CA.

Maybe the Gardenlet in the soil could copy the Kubelet CA to the kube-system
namespace of its child shoot, and the Gardenlet in the seed, if it finds a
ca-kubelet secret in the kube-system namespace (i.e. a managed seed), could copy
it to the garden namespace. A conditional Prometheus volume could be used to
mount this ca-kubelet to Prometheus if it exists (managed seed) or mount
kube-root-ca.crt otherwise. This condition handling has to happen in Gardener
because Prometheus expects a single CA in its configuration.

> KEP-3257: Cluster Trust Bundles
> https://github.com/kubernetes/enhancements/tree/master/keps/sig-auth/3257-cluster-trust-bundles
could be also a good approach to make the Kubelet CA-s available
for the workload.

The cluster internal communication is still encrypted (protection against
a passively eavesdropping entity), but the certificate is not verified,
which could allow for a man in the middle attack by an active entity.

For the cluster internal scraping of the kubelet metrics, this might
be acceptable.

Both the ideas above might not be feasible short term, so with this PR,
the certificate verification is skipped.

pkg/component/observability/monitoring/prometheus/cache/assets/scrapeconfigs/cadvisor.yaml

@@ -6,6 +6,7 @@ metrics_path: /metrics/cadvisor
 
 tls_config:
   ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+  insecure_skip_verify: true
 bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
 
 kubernetes_sd_configs:

pkg/component/observability/monitoring/prometheus/cache/assets/scrapeconfigs/kubelet.yaml

@@ -4,6 +4,7 @@ scheme: https
 
 tls_config:
   ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+  insecure_skip_verify: true
 bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
 
 kubernetes_sd_configs:

pkg/component/observability/monitoring/prometheus/cache/scrapeconfigs_test.go

@@ -23,6 +23,7 @@ metrics_path: /metrics/cadvisor
 
 tls_config:
   ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+  insecure_skip_verify: true
 bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
 
 kubernetes_sd_configs:
@@ -83,6 +84,7 @@ scheme: https
 
 tls_config:
   ca_file: /var/run/secrets/kubernetes.io/serviceaccount/ca.crt
+  insecure_skip_verify: true
 bearer_token_file: /var/run/secrets/kubernetes.io/serviceaccount/token
 
 kubernetes_sd_configs: