Multiple static routes inside one private gateway.

### problem

As an admin I want to have possibility to add more than one static route to private gateway.
Unfortunately, I'm not sure if this a bug or a feature.

But if I try to add the second static route (Destination Network CIDR) to Private Gateway, just nothing happens after I push the "Add route" button. Screenshot with example is attached.


### versions

The versions of ACS, hypervisors, storage, network etc..
Current ACS version is 4.20.0.0
Previous vith the same issue: 4.19.1
Hypervisors: XCP-NG 8.2.1

### The steps to reproduce the bug

1. Create new Private Pateway
2. Add first Destination Network CIDR
3. Add second Destination Network CIDR

### What to do about it?

_No response_

Reproduce the issue in CloudStack 4.20.0.0 by creating a private gateway, adding one Destination Network CIDR, and submitting a second through the “Add route” control. Trace the private-gateway route-add entry point and verify that multiple routes are accepted and the second CIDR is persisted or reported with a visible error.

Written by the indexing model from the issue text.