Disable Sync share with group lock for ProjectNamespaces

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.

Disables the before_create/update callbacks for a method that only makes sense for Group type namespaces, not ProjectNamespace objects.

Third bit of #341823 (closed)

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Related to #341823 (closed)