Fix the problem of not being able to resolve headers correctly (#6786)

#### What type of PR is this?

/kind bug
/area core
/milestone 2.20.x

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

This PR changes server.forward-header-strategy to native instead of framework due to a bug of Spring Framework 6.20.0-RC.1.

See spring-projects/spring-framework#32097 (comment) for more.

If Halo server is proxied by OpenResty which is using HTTP 2, all header names proxied into Halo server will be lowercase. This behavior makes Halo get a null header(e.g.:: `content-type: application/json`) while invoking `request.getHeaders().getContentType()`.

And I found that `ServerHttpRequest` is mutated by `org.springframework.web.server.adapter.ForwardedHeaderTransformer`, so I try to use native forward-header-strategy to resolve the problem and it works very well. See [reactor.netty.http.server.DefaultHttpForwardedHeaderHandler](https://github.com/reactor/reactor-netty/blob/446683826b3020782fe3d647ef264ca7eace94f6/reactor-netty-http/src/main/java/reactor/netty/http/server/DefaultHttpForwardedHeaderHandler.java) for more.

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

```release-note
None
```

application/src/main/resources/application.yaml

@@ -1,6 +1,6 @@
 server:
   port: 8090
-  forward-headers-strategy: framework
+  forward-headers-strategy: native
   compression:
     enabled: true
   error:

application/src/test/java/run/halo/app/XForwardHeaderTest.java

@@ -17,7 +17,7 @@
 import reactor.test.StepVerifier;
 
 @SpringBootTest(webEnvironment = RANDOM_PORT,
-    properties = "server.forward-headers-strategy=framework")
+    properties = "server.forward-headers-strategy=native")
 class XForwardHeaderTest {
 
     @LocalServerPort