Change sample build to use java11 container

Author: cindy-peng

.cloudbuild/samples_build.yaml

@@ -1,19 +1,19 @@
 steps:
-- name: gcr.io/cloud-devrel-public-resources/java8
+- name: gcr.io/cloud-devrel-public-resources/java11
   entrypoint: ls
   args: [
     '-alt',
   ]
-- name: gcr.io/cloud-devrel-public-resources/java8
+- name: gcr.io/cloud-devrel-public-resources/java11
   entrypoint: curl
   args: [
     '--header',
     'Metadata-Flavor: Google',
     'http://metadata.google.internal/computeMetadata/v1/instance/service-accounts/default/email'
   ]
-- name: gcr.io/cloud-devrel-public-resources/java8
+- name: gcr.io/cloud-devrel-public-resources/java11
   entrypoint: pwd
-- name: gcr.io/cloud-devrel-public-resources/java8
+- name: gcr.io/cloud-devrel-public-resources/java11
   entrypoint: bash
   args: [
     '.kokoro/build.sh'
@@ -22,8 +22,7 @@ steps:
   - 'JOB_TYPE=samples'
   - 'GOOGLE_CLOUD_PROJECT=cloud-java-ci-sample'
   - 'KOKORO_GITHUB_PULL_REQUEST_NUMBER=$_PR_NUMBER'
-  - 'JAVA=/usr/lib/jvm/java-11-openjdk-amd64/bin/java'
-- name: gcr.io/cloud-devrel-public-resources/java8
+- name: gcr.io/cloud-devrel-public-resources/java11
   entrypoint: echo
   args: [
     'Sample job succeeded',