Switch CI to use Java 21

[lauraharker]

Should fix

  stderr: 'Error: LinkageError occurred while loading main class com.google.javascript.jscomp.CommandLineRunner\n' +
    '\tjava.lang.UnsupportedClassVersionError: com/google/javascript/jscomp/CommandLineRunner has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 61.0\n',
  exitCode: 1

from https://github.com/google/closure-compiler/actions/runs/12676540151/job/35331314620

#4209