When running Jenkins with Java 17, one needs to pass in several --add-opens directives on the command line when running java -jar jenkins.war. It is error-prone to do this everywhere we run java -jar jenkins.war. Instead we should add an Add-Opens entry to MANIFEST.MF instead. Adding this entry has no impact on older versions of Java, so why not do it now? This just makes it easier to run Jenkins on Java 17.