Test Java 21 deployment

Update runtime to Java 21

This PR makes the runtime of most of our workload Java 21.

1. App Engine. Java 21 is in GA and it supports Java EE 8. I had to add
   an environmental variable so that we don't get an
   AppEngineCredentails by default (we have been using
   ComputeEngineCredentials for a couple of years). The uprade to Java
   21 runtime changed a system property that controls how jetty logging
   works, which also control if AppEngineCredential is return. Tested by
   deploying to alpha.
2. Proxy base image upgarded to Java 21 (distroless still doesn't
   support Java 21 and it looks like Temurin is the way to go
   b/306728455). Tested by deploying to alpha.
3. Beam pipeline base image upgrade to Java 21. The JAVA21 flag is not
   supported by gcloud yet, but specifying the image URL directly works
   (and is supported). Tested by running in alpha.
4. Jetty base image upgraded to Java 21. Tested locally.