[network] Difference about load balancer rule between api verison '2020-11-01' and '2021-02-01'

https://github.com/Azure/azure-rest-api-specs/blob/f15b4f3299df66c755156aaea2c3f0c64f1840bb/specification/network/resource-manager/Microsoft.Network/stable/2021-02-01/loadBalancer.json#L1756-L1762

'backendAddressPools' is the new definition and it will be set with same value with 'backendAddressPool' by service if users don't specify it.
(e-mail : RE: CLI for Gateway LB)

Service explanation:
Having BackendAddressPools is expected from Lb Rule. And they should be the same.

The difference between BackendAddressPool and BackendAddressPools is that only for our new feature, Gateway LB, we use BackendAddressPools where it can be multiple.
For non-Gateway LB scenarios, users will not use BackendAddressPools at all.

So for users, they can have inside LB Rule:
1. Empty BackendAddressPool and filled BackendAddressPools (1 or 2 BackendAddressPool)
2. Filled BackendAddressPool and a single BackendAddressPool in BackendAddressPools. Both should be exactly the same (I think this is where the error is coming from. Our service should allow when you only update BackendAddressPool)

Start with the linked loadBalancer.json lines 1756-1762 and compare the corresponding load balancer rule definitions in API versions 2020-11-01 and 2021-02-01. Trace the relationship between BackendAddressPool and BackendAddressPools, then confirm the intended behavior from the service explanation. Done means the specification and the described update behavior agree.

Written by the indexing model from the issue text.