Availability of Network Offering not taken into account in Web Interface

### problem

When loading the CreateIsolatedNetworkForm form, the availability is not taken into account in selecting the default network offering.
It seems the issue is also present in CreateSharedNetworkForm and CreateL2NetworkForm

### versions

Running 4.22.0 but issue still present in main branch

### The steps to reproduce the bug

1. Add network
2. Check if the pre-selected network is the one with Required availability

### What to do about it?

https://github.com/apache/cloudstack/blob/6dc259c7dab7bee0c4ff731271c61e2f2ee97532/ui/src/views/network/CreateIsolatedNetworkForm.vue#L565C1-L565C69
Change this line to select the first element with the availability "Required" instead of the first of the list (That is sorted by ID by default)

从 ui/src/views/network/CreateIsolatedNetworkForm.vue 第 565 行附近开始，然后检查 CreateSharedNetworkForm 和 CreateL2NetworkForm 中相应的默认选择逻辑。重现 Add network 流程，并验证预选的 offering 是第一个具有 Required availability 的 offering，而不是 ID 排序列表中的第一个 offering。

由索引模型根据 Issue 内容生成。