Allow other "timestamp" types in MVC error model

Remove casting "timestamp" to `java.util.Date` in
`ErrorMvcAutoConfiguration` as the cast is not necessary and it
prevents other types (e.g. `java.time`) from being used.

See gh-23256

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/error/ErrorMvcAutoConfiguration.java

@@ -206,7 +206,7 @@ public class ErrorMvcAutoConfiguration {
 			}
 			response.setContentType(TEXT_HTML_UTF8.toString());
 			StringBuilder builder = new StringBuilder();
-			Date timestamp = (Date) model.get("timestamp");
+			Object timestamp = model.get("timestamp");
 			Object message = model.get("message");
 			Object trace = model.get("trace");
 			if (response.getContentType() == null) {