Fix the problem of updating plugin extensions incorrectly (#5404)

#### What type of PR is this?

/kind bug
/area plugin
/area core
/milestone 2.13.x

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

Fix the problem of updating plugin extensions incorrectly.

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

Fixes https://github.com/halo-dev/halo/issues/5394

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

```release-note
None
```

application/src/main/java/run/halo/app/plugin/PluginStartedListener.java

@@ -38,6 +38,7 @@ public class PluginStartedListener {
             .doOnNext(old -> {
                 unstructured.getMetadata().setVersion(old.getMetadata().getVersion());
             })
+            .map(ignored -> unstructured)
             .flatMap(client::update)
             .switchIfEmpty(Mono.defer(() -> client.create(unstructured)));
     }