improve the sorting rules for themes side posts using publishTime (#3148)

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#### What type of PR is this?
/kind improvement
/area core
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#### What this PR does / why we need it:
Provide administrators with the ability to adjust the order of articles by modifying the publish time.

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

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Fixes https://github.com/halo-dev/halo/issues/3143

#### Special notes for your reviewer:
修改前：
![image](https://user-images.githubusercontent.com/28662535/212074405-e0a351fe-9293-4a4f-af9c-41f720512f9b.png)
修改后：
![image](https://user-images.githubusercontent.com/28662535/212074498-1091bb62-700e-4e1f-872a-46687cdbd81c.png)

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

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```release-note
主题侧文章以发布时间排序
```

src/main/java/run/halo/app/theme/finders/impl/PostFinderImpl.java

@@ -373,12 +373,12 @@ public class PostFinderImpl implements PostFinder {
             post -> Objects.requireNonNullElse(post.getSpec().getPinned(), false);
         Function<Post, Integer> priority =
             post -> Objects.requireNonNullElse(post.getSpec().getPriority(), 0);
-        Function<Post, Instant> creationTimestamp =
-            post -> post.getMetadata().getCreationTimestamp();
+        Function<Post, Instant> publishTime =
+            post -> post.getSpec().getPublishTime();
         Function<Post, String> name = post -> post.getMetadata().getName();
         return Comparator.comparing(pinned)
             .thenComparing(priority)
-            .thenComparing(creationTimestamp)
+            .thenComparing(publishTime, Comparators.nullsLow())
             .thenComparing(name)
             .reversed();
     }

src/test/java/run/halo/app/theme/finders/impl/PostFinderImplTest.java

@@ -193,35 +193,35 @@ class PostFinderImplTest {
 
     List<Post> posts() {
         // 置顶的排前面按 priority 排序
-        // 再根据创建时间排序
+        // 再根据发布时间排序
         // 相同再根据名称排序
         // 6, 2, 1, 5, 4, 3
         Post post1 = post(1);
         post1.getSpec().setPinned(false);
-        post1.getMetadata().setCreationTimestamp(Instant.now().plusSeconds(20));
+        post1.getSpec().setPublishTime(Instant.now().plusSeconds(20));
 
         Post post2 = post(2);
         post2.getSpec().setPinned(true);
         post2.getSpec().setPriority(2);
-        post2.getMetadata().setCreationTimestamp(Instant.now());
+        post2.getSpec().setPublishTime(Instant.now());
 
         Post post3 = post(3);
         post3.getSpec().setDeleted(true);
-        post3.getMetadata().setCreationTimestamp(Instant.now());
+        post3.getSpec().setPublishTime(Instant.now());
 
         Post post4 = post(4);
         post4.getSpec().setVisible(Post.VisibleEnum.PRIVATE);
-        post4.getMetadata().setCreationTimestamp(Instant.now());
+        post4.getSpec().setPublishTime(Instant.now());
 
         Post post5 = post(5);
         post5.getSpec().setPublish(false);
         post5.getMetadata().getLabels().clear();
-        post5.getMetadata().setCreationTimestamp(Instant.now());
+        post5.getSpec().setPublishTime(Instant.now());
 
         Post post6 = post(6);
         post6.getSpec().setPinned(true);
         post6.getSpec().setPriority(3);
-        post6.getMetadata().setCreationTimestamp(Instant.now());
+        post6.getSpec().setPublishTime(Instant.now());
 
         return List.of(post1, post2, post3, post4, post5, post6);
     }