[system.err] Error: Could not find or load main class org.sonar.runner.Main

I only see the class org/sonar/runner/cli/Main.class which doesn’t line up with the hard-coded class name in the plugin. That’s why it is failing for you. The plugin code is not compatible with the library version anymore.

BTW: The SonarRunner plugin in Gradle core is superseded by an external plugin. Please use the external plugin. They might have fixed the issue.