Support building with JDK 21 (#234)

- Update google-java-format to a version compatible with JDK 21 and run "gw spotlessApply".
- Fix wrong test assumption
  JavaCodeGenerator writes a properties file using java.util.Properties,
  which doesn't guarantee order of entries.
- Fix most deprecation warnings
- Add CI job for JDK 21

pkl-tools/src/dummy/java/org/pkl/tools/Empty.java

@@ -20,5 +20,6 @@ package org.pkl.tools;
  */
 // TODO: figure out how to generate javadoc for a shadow jar.
 @SuppressWarnings("unused")
-public class Empty {
+public final class Empty {
+  private Empty() {}
 }