[#11080] Support initial deployment to Java 11 runtime

[wkurniawan07]

Fixes #11080

This will wrap up the first milestone of GAE runtime upgrade.

After this PR:

• The local development environment is completely and entirely decoupled from App Engine, as evident from the commit which removes GaeSimulation (there is no GAE to be simulated anymore) and the commit that removes all App Engine dependencies.
  • Consequently, all documentation references to legacy GAE integration including usage of App Engine * APIs are updated.
  • The Cloud Code plugin for IntelliJ is no longer needed and this makes the IntelliJ setup guide moot, as it becomes a generic import-project-to-workspace instructions. As such, the guide is removed. However, we are now confident to claim that any IDE supporting Java/Gradle development will work.
• GAE only plays a part in when deployment. This means GAE's role is now strictly platform-as-a-service, and rightly so.
• Deployment will be targeted to Java 11 a.k.a. GAE second generation runtime.

[Derek-Hardy]

LGTM!