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)

從 ui/src/views/network/CreateIsolatedNetworkForm.vue 約第 565 行開始，然後檢查 CreateSharedNetworkForm 和 CreateL2NetworkForm 中對應的預設選取邏輯。重現 Add network 流程，並確認預先選取的 offering 是第一個具有 Required availability 的 offering，而不只是依 ID 排序之清單中的第一個 offering。

由索引模型根據 Issue 內容生成。