Upgrade to Java 25 (#3000)

Error-prone introduced many more checks in Java 25. We fixed a few
and suppressed most. A follow-up bug is opened to clean this up.
An ai agent should be able to clean up most of it.

This PR is created with gemini-cli. Summary of experience:

* The good: AI caught most compatibility issues, and with permission,
  suppressed them through compiler flags and errorprone options.
  It also caught many versio references in scripts.

* Where it didn't shine:
  - It did not find and update the target version spec in the custome
    VKey annotation processor source file.
  - It did not flag eclipse-temurin:21 docker image for upgrade.
  - When running into failure, its first instinct is to disable checks
    e.g., -Werror instead of fixing them.

util/src/main/java/google/registry/util/GoogleCredentialsBundle.java

@@ -53,8 +53,8 @@ public class GoogleCredentialsBundle implements Serializable {
    * possible.
    */
   public String serviceAccount() {
-    if (googleCredentials instanceof ServiceAccountSigner) {
-      return ((ServiceAccountSigner) googleCredentials).getAccount();
+    if (googleCredentials instanceof ServiceAccountSigner signer) {
+      return signer.getAccount();
     } else {
       throw new RuntimeException(
           String.format(