SREP-1565 - Remove cluster-admins exception from namespaces webhook

[tnierman]

https://issues.redhat.com/browse/SREP-1565

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Removes an exception in the namespaces webhook which allows members of the cluster-admins group, the group customer users are added to when granted cluster-admin privileges in OCM, to modify platform-namespaces and privileged labels which can impact their SLA. It does not impact backplane-cluster-admin group members nor layered-sre-cluster-admins.

[clcollins]

/lgtm
/approve

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