fix: serde with custom problem

related to: Micronaut Serde Jackson with @JsonUnwrapped and @JsonIgnoreProperties doesn't work #233
see: micronaut-projects/micronaut-serialization#233

Compileation fails:

```
$ /problem-json/src/main/java/io/micronaut/problem/ProblemErrorResponseProcessor.java:136: error: Unwrapped property contains a property [type] that conflicts with an existing property of the outer type: io.micronaut.problem.ProblemErrorResponseProcessor$ThrowableProblemWithoutStacktrace. Consider specifying a prefix or suffix to disambiguate this conflict.
        final ThrowableProblem problem;
                               ^
```

problem-json/src/main/java/io/micronaut/problem/ProblemErrorResponseProcessor.java

@@ -17,7 +17,6 @@
 
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonUnwrapped;
-import io.micronaut.core.annotation.Introspected;
 import io.micronaut.core.annotation.NonNull;
 import io.micronaut.http.HttpMethod;
 import io.micronaut.http.HttpStatus;
@@ -27,6 +26,7 @@
 import io.micronaut.http.server.exceptions.response.ErrorResponseProcessor;
 import io.micronaut.problem.conf.ProblemConfiguration;
 import io.micronaut.problem.conf.ProblemConfigurationProperties;
+import io.micronaut.serde.annotation.Serdeable;
 import io.micronaut.web.router.exceptions.UnsatisfiedRouteException;
 import jakarta.inject.Inject;
 import org.zalando.problem.Problem;
@@ -129,7 +129,7 @@ protected boolean includeErrorMessage(@NonNull ErrorContext errorContext) {
                 .orElse(false);
     }
 
-    @Introspected
+    @Serdeable
     static final class ThrowableProblemWithoutStacktrace implements Problem {
         @JsonUnwrapped
         @JsonIgnoreProperties(value = {"stackTrace", "localizedMessage", "message", "type", "title", "status", "detail", "instance", "parameters"})

test-suite-serde-java/src/test/java/io/micronaut/problem/ProblemCustomTest.java

@@ -13,7 +13,6 @@
 import io.micronaut.serde.annotation.Serdeable;
 import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
 import jakarta.inject.Inject;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import org.zalando.problem.AbstractThrowableProblem;
 
@@ -31,7 +30,6 @@ class ProblemCustomTest {
     HttpClient httpClient;
 
     @Test
-    @Disabled("pending fix for https://github.com/micronaut-projects/micronaut-problem-json/issues/176")
     void customProblemAreSerializedWithSerde() {
         HttpClientResponseException e = assertThrows(HttpClientResponseException.class,
             () -> httpClient.toBlocking().exchange(HttpRequest.GET("/product/problem"))