fix: saving an post causes the publish time to be lost (#809)

#### What type of PR is this?

/kind bug

#### What this PR does / why we need it:

修复保存文章设置导致发布时间丢失的问题。

see #807

#### Which issue(s) this PR fixes:

Fixes halo-dev/halo#3080

#### Special notes for your reviewer:

测试方式：

1. 创建若干文章。
2. 返回到文章管理列表。
3. 打开任意文章的设置。
4. 检查发布时间是否回显，然后保存。
5. 保存之后检查发布时间是否还存在。

#### Does this PR introduce a user-facing change?

```release-note
修复 Console 端保存文章设置导致发布时间丢失的问题。
```

src/modules/contents/pages/components/SinglePageSettingModal.vue

@@ -277,7 +277,7 @@ const onPublishTimeChange = (value: string) => {
             type="select"
           ></FormKit>
           <FormKit
-            :value="publishTime"
+            :model-value="publishTime"
             label="发表时间"
             type="datetime-local"
             name="publishTime"

src/modules/contents/posts/components/PostSettingModal.vue

@@ -279,7 +279,7 @@ const onPublishTimeChange = (value: string) => {
             type="select"
           ></FormKit>
           <FormKit
-            :value="publishTime"
+            :model-value="publishTime"
             label="发表时间"
             type="datetime-local"
             @input="onPublishTimeChange"