fix: Remove unrequired availability_domain from vnic_attachments data

[devoncrouse]

The availability_domain parameter isn't required for oci_core_vnic_attachments, and a changed value currently yields an error on apply when trying to evaluate the data source, though the intention is to ignore lifecycle changes:

│ Error: Invalid index                                                                                                                                                                                     
│                                                                                                                                                                                                          
│   on .terraform/modules/operator/datasources.tf line 64, in data "oci_core_vnic" "operator_vnic":                                                                                                        
│   64:   vnic_id    = lookup(data.oci_core_vnic_attachments.operator_vnics_attachments.vnic_attachments[0], "vnic_id")                                          
│     ├────────────────                                                                                                                                                                                    
│     │ data.oci_core_vnic_attachments.operator_vnics_attachments.vnic_attachments is empty list of object                                                        
│                                                                                                    
│ The given key does not identify an element in this collection value: the collection has no elements.

[hyder]

Approved