fix: Issue #2305 - Fix diff recognition for removal of network security group IDs in oci_apigateway_gateway

[rvajustin]

Summary:

This PR addresses an issue where updating an API Gateway to remove all associated network security group IDs does not trigger a change in Terraform. The resource update logic fails to send an empty list for network_security_group_ids when they are removed, resulting in the NSG remaining attached.

Changes:

• Update Handling: Modified both the create and update functions so that the network security group IDs field is always assigned to the update request.
• Explicit Change Detection: In the update function, the logic now checks with s.D.HasChange("network_security_group_ids") and explicitly assigns the (possibly empty) list to request.NetworkSecurityGroupIds.
• Consistency: These changes ensure that when the NSG IDs are removed from the configuration, an empty list is sent to the API, prompting the correct removal of the association.

Testing:

1. Create an API Gateway with a non-empty network_security_group_ids.
2. Update the Terraform configuration to remove the NSG IDs.
3. Execute terraform plan and terraform apply to verify that the NSG is properly removed from the API Gateway.

This fix ensures that the Terraform provider correctly detects the removal of NSG IDs and aligns the actual state of the API Gateway with the desired configuration.

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