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

.circleci/config.pkl

@@ -148,6 +148,10 @@ local gradleCheckJobs: Mapping<String, GradleCheckJob> = new {
     javaVersion = "17.0"
     isRelease = false
   }
+  ["gradle-check-jdk21"] {
+    javaVersion = "21.0"
+    isRelease = false
+  }
 }
 
 jobs {