Fix random list problem of tags and categories (#3261)

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#### What type of PR is this?
/kind bug

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#### What this PR does / why we need it:
replaced flatMap with flatMapSequential to ensure that the list of tags and categories shown in console post order is not random

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

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Fixes #3139

#### Special notes for your reviewer:
/cherrypick release-2.0
/cherrypick release-2.1

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

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```release-note
修复控制台文章列表中，标签和分类顺序随机的问题
```

src/main/java/run/halo/app/content/impl/PostServiceImpl.java

@@ -208,7 +208,7 @@ public class PostServiceImpl extends AbstractContentService implements PostServi
             return Flux.empty();
         }
         return Flux.fromIterable(tagNames)
-            .flatMap(tagName -> client.fetch(Tag.class, tagName));
+            .flatMapSequential(tagName -> client.fetch(Tag.class, tagName));
     }
 
     private Flux<Category> listCategories(List<String> categoryNames) {
@@ -216,7 +216,7 @@ public class PostServiceImpl extends AbstractContentService implements PostServi
             return Flux.empty();
         }
         return Flux.fromIterable(categoryNames)
-            .flatMap(categoryName -> client.fetch(Category.class, categoryName));
+            .flatMapSequential(categoryName -> client.fetch(Category.class, categoryName));
     }
 
     private Flux<Contributor> listContributors(List<String> usernames) {