Use the platform-specific line separator

See gh-11674

spring-boot-test/src/main/java/org/springframework/boot/test/json/DuplicateJsonObjectContextCustomizerFactory.java

@@ -75,13 +75,13 @@ class DuplicateJsonObjectContextCustomizerFactory implements ContextCustomizerFa
 		}
 
 		private void logDuplicateJsonObjectsWarning(List<URL> jsonObjects) {
-			StringBuilder message = new StringBuilder("\n\nFound multiple occurrences of"
-					+ " org.json.JSONObject on the class path:\n\n");
+			StringBuilder message = new StringBuilder(String.format("%n%nFound multiple occurrences of"
+					+ " org.json.JSONObject on the class path:%n%n"));
 			for (URL jsonObject : jsonObjects) {
-				message.append("\t" + jsonObject + "\n");
+				message.append(String.format("\t%s%n", jsonObject));
 			}
-			message.append("\nYou may wish to exclude one of them to ensure"
-					+ " predictable runtime behaviour\n");
+			message.append(String.format("%nYou may wish to exclude one of them to ensure"
+					+ " predictable runtime behavior%n"));
 			this.logger.warn(message);
 		}