Update default Secrets name for system users

[rajaie7]

seems the current docs reference an incorrect default secret name for system users. https://github.com/percona/percona-server-mongodb-operator/blob/59bf8a93133323f37bf0688d47f56cb16432f850/pkg/apis/psmdb/v1/psmdb_defaults.go#L47

[egegunes]

@rajaie7 the default secrets name is <cr.Name>-secrets. Example cr.yaml in our repo is named my-cluster-name so my-cluster-name-secrets is correct.

Yes, if you don't specify secrets.users, the default name is percona-server-mongodb-secrets. I think this is wrong and we need to change the default name to <cr.Name>-secrets to be consistent across operators.