Fix the problem of listing users with unstable result (#3635)

#### What type of PR is this?

/kind bug
/area core

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

Use concatMap instead of flatMap to ensure stable result. 

Please see the differences between flatMap and concatMap from links below:

- https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html#flatMap-java.util.function.Function-
- https://projectreactor.io/docs/core/release/api/reactor/core/publisher/Flux.html#concatMap-java.util.function.Function-

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

Fixes https://github.com/halo-dev/halo/issues/3555

#### Special notes for your reviewer:

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

```release-note
修复用户列表排序不稳定问题
```

application/src/main/java/run/halo/app/core/extension/endpoint/UserEndpoint.java

@@ -397,7 +397,7 @@ public class UserEndpoint implements CustomEndpoint {
 
     private Mono<ListResult<ListedUser>> toListedUser(ListResult<User> listResult) {
         return Flux.fromStream(listResult.get())
-            .flatMap(user -> {
+            .concatMap(user -> {
                 Set<String> roleNames = roleNames(user);
                 return roleService.list(roleNames)
                     .collectList()