DC names are not properly sanitized

[olim7t]

If a cluster declares a datacenter with invalid characters:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  namespace: k8ssandra-operator
  name: demo
spec:
  cassandra:
    serverVersion: 4.1.0
    datacenters:
      - metadata:
          name: dc_1   # <<< invalid Kubernetes name
        size: 1
        racks:
          - name: rack1
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: standard
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 100Mi

The name is not properly sanitized, which leads to this error:

ConfigMap "demo-dc_1-metrics-agent-config" is invalid: metadata.name: Invalid value: "demo-dc_1-metrics-agent-config": a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')

The problem is not limited to that ConfigMap: even after fixing it (here), the invalid name causes another error when the operator tries to create the CassandraDatacenter.

We should either reject invalid DC names in the validation webhook, or sanitize them and maybe use the original value as the datacenterName.