fix: issue of abnormal sorting in the aggregation query interface (#2623)

#### What type of PR is this?

/kind bug
/milestone 2.0

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

修复文章、自定义页面、评论聚合查询接口排序不固定的问题。

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

Fixes https://github.com/halo-dev/halo/issues/2622

#### Special notes for your reviewer:

测试方式：检查 Console 端的文章、自定义页面、评论、回复的排序是否正常，需要多次刷新检查。

```release-note
修复文章、自定义页面、评论聚合查询接口排序不固定的问题。
```

src/main/java/run/halo/app/content/comment/CommentServiceImpl.java

@@ -52,7 +52,7 @@ public class CommentServiceImpl implements CommentService {
                 commentQuery.getPage(), commentQuery.getSize())
             .flatMap(comments -> Flux.fromStream(comments.get()
                     .map(this::toListedComment))
-                .flatMap(Function.identity())
+                .concatMap(Function.identity())
                 .collectList()
                 .map(list -> new ListResult<>(comments.getPage(), comments.getSize(),
                     comments.getTotal(), list)

src/main/java/run/halo/app/content/comment/ReplyServiceImpl.java

@@ -99,7 +99,7 @@ public class ReplyServiceImpl implements ReplyService {
                 query.getPage(), query.getSize())
             .flatMap(list -> Flux.fromStream(list.get()
                     .map(this::toListedReply))
-                .flatMap(Function.identity())
+                .concatMap(Function.identity())
                 .collectList()
                 .map(listedReplies -> new ListResult<>(list.getPage(), list.getSize(),
                     list.getTotal(), listedReplies))

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

@@ -69,7 +69,7 @@ public class PostServiceImpl implements PostService {
             .flatMap(listResult -> Flux.fromStream(
                         listResult.get().map(this::getListedPost)
                     )
-                    .flatMap(Function.identity())
+                    .concatMap(Function.identity())
                     .collectList()
                     .map(listedPosts -> new ListResult<>(listResult.getPage(), listResult.getSize(),
                         listResult.getTotal(), listedPosts)

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

@@ -70,7 +70,7 @@ public class SinglePageServiceImpl implements SinglePageService {
             .flatMap(listResult -> Flux.fromStream(
                         listResult.get().map(this::getListedSinglePage)
                     )
-                    .flatMap(Function.identity())
+                    .concatMap(Function.identity())
                     .collectList()
                     .map(listedSinglePages -> new ListResult<>(listResult.getPage(),
                         listResult.getSize(),

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

@@ -49,7 +49,7 @@ public class CommentFinderImpl implements CommentFinder {
                 defaultComparator(),
                 pageNullSafe(page), sizeNullSafe(size))
             .flatMap(list -> Flux.fromStream(list.get().map(this::toCommentVo))
-                .flatMap(Function.identity())
+                .concatMap(Function.identity())
                 .collectList()
                 .map(commentVos -> new ListResult<>(list.getPage(), list.getSize(), list.getTotal(),
                     commentVos)
@@ -68,7 +68,7 @@ public class CommentFinderImpl implements CommentFinder {
                     && Objects.equals(true, reply.getSpec().getApproved()),
                 comparator.reversed(), pageNullSafe(page), sizeNullSafe(size))
             .flatMap(list -> Flux.fromStream(list.get().map(this::toReplyVo))
-                .flatMap(Function.identity())
+                .concatMap(Function.identity())
                 .collectList()
                 .map(replyVos -> new ListResult<>(list.getPage(), list.getSize(), list.getTotal(),
                     replyVos))