Allow superuserSecretName to be updated

[jsanda]

What is missing?
The validating webhook does not allow the superuserSecretName to be updated.

Why do we need it?
In k8ssandra/k8ssandra-operator#566 a user reported a problem not being able to change the superuser secret reference. While investigating the issue I noticed that the validating webhook in cass-operator does not allow the secret reference to be updated.

This is necessary to support revocation and rotation policies.

This may get into some Cassandra user/role management which the operator doesn't really do, so I think we should carefully consider how to address this.

Environment

• Cass Operator version:

  v1.11.0

  **Anything else we need to know?**:

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: CASS-37