Do not validate ProjectNamespaces for hierarchy

What does this MR do and why?

Some scopes in Namespaces, particularly around Group/subgroup hierarchies, are called by logic that expects these objects to be Group objects themselves.

This MR filters ProjectNamespace objects from the the certain validations that only make sense when validating a group object.

Third bit of #341823 (closed)

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Related to #341823 (closed)