Fix stack-trace line breaks in whitelabel error views

Since gh-12838, the WebFlux whitelabel error view also shows the
stacktrace information if it is available in the model.

This commit adds a CSS directive that preserves white spaces and breaks
lines at newline characters.

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/AbstractErrorWebExceptionHandler.java

@@ -202,7 +202,7 @@ public abstract class AbstractErrorWebExceptionHandler
 			builder.append("<div>").append(htmlEscape(message)).append("</div>");
 		}
 		if (trace != null) {
-			builder.append("<div>").append(htmlEscape(trace)).append("</div>");
+			builder.append("<div style='white-space:pre-wrap;'>").append(htmlEscape(trace)).append("</div>");
 		}
 		builder.append("</body></html>");
 		return responseBody.syncBody(builder.toString());