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_

在 CloudStack 4.20.0.0 中重現此問題：建立一個私有閘道，新增一個 Destination Network CIDR，然後透過「Add route」控制項提交第二個 CIDR。追蹤私有閘道的路由新增入口點，並確認是否接受多個路由，以及第二個 CIDR 是否已持久化，或是否回報可見的錯誤。

由索引模型根據 Issue 內容生成。