mgr/dashboard: disable cluster selection in NFS export editing form

mgr/dashboard: disable cluster selection in NFS export editing form

We should not allow changing an export's cluster because an export ID
might live in one cluster but not in another one. Editing a non-existing
export in a cluster causes an error.

Fixes: https://tracker.ceph.com/issues/47373
Signed-off-by: Kiefer Chang <kiefer.chang@suse.com>
(cherry picked from commit d678d8076c2a4c5edfe489d553e3c8770462f023)

 Conflicts:
	src/pybind/mgr/dashboard/frontend/src/app/ceph/nfs/nfs-form/nfs-form.component.spec.ts
        - Unnecessary spies for SummaryService are removed in the master.
        - The LoadingPanelComponent is new in the master.