Upgrade to Checkstyle 10.21.4

Author: jhoeller

buildSrc/src/main/java/org/springframework/build/CheckstyleConventions.java

@@ -50,7 +50,7 @@ public void apply(Project project) {
 			project.getPlugins().apply(CheckstylePlugin.class);
 			project.getTasks().withType(Checkstyle.class).forEach(checkstyle -> checkstyle.getMaxHeapSize().set("1g"));
 			CheckstyleExtension checkstyle = project.getExtensions().getByType(CheckstyleExtension.class);
-			checkstyle.setToolVersion("10.21.2");
+			checkstyle.setToolVersion("10.21.4");
 			checkstyle.getConfigDirectory().set(project.getRootProject().file("src/checkstyle"));
 			String version = SpringJavaFormatPlugin.class.getPackage().getImplementationVersion();
 			DependencySet checkstyleDependencies = project.getConfigurations().getByName("checkstyle").getDependencies();