Gradle-wrapper.jar not updated from 6.0.1 to 6.3

You’re not referencing the correct JAR there. The classes in lib/gradle-wrapper-6.3.jar are on the classpath, but not that JAR itself. The lib/plugins/gradle-plugins-6.3.jar contains the gradle-wrapper.jar which is used by the wrapper task as a classpath resource to copy to the project folder.