fix(java): apache-httpclient serialization error

fixes following related issue:
OpenAPITools#12797

modules/openapi-generator/src/main/resources/Java/libraries/apache-httpclient/ApiClient.mustache

@@ -61,6 +61,7 @@ import java.io.InputStream;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.nio.charset.UnsupportedCharsetException;
 import java.nio.file.Files;
 import java.nio.file.StandardCopyOption;
@@ -785,7 +786,7 @@ public class ApiClient{{#jsr310}} extends JavaTimeFormatter{{/jsr310}} {
     String mimeType = contentType.getMimeType();
     if (isJsonMime(mimeType)) {
       try {
-        return new StringEntity(objectMapper.writeValueAsString(obj), contentType);
+        return new StringEntity(objectMapper.writeValueAsString(obj), contentType.withCharset(StandardCharsets.UTF_8));
       } catch (JsonProcessingException e) {
         throw new ApiException(e);
       }