Update lint check to use Java 11 image. (#3192)

Fixes #issue

> It's a good idea to open an issue first for discussion.

- [ ] I have followed [Sample Format Guide](https://github.com/GoogleCloudPlatform/java-docs-samples/blob/master/SAMPLE_FORMAT.md)
- [x] `pom.xml` parent set to latest `shared-configuration`
- [x] Appropriate changes to README are included in PR
- [ ] API's need to be enabled to test (tell us)
- [ ] Environment Variables need to be set (ask us to set them)
- [x] **Tests** pass:   `mvn clean verify` **required**
- [x] **Lint**  passes: `mvn -P lint checkstyle:check` **required**
- [ ] **Static Analysis**:  `mvn -P lint clean compile pmd:cpd-check spotbugs:check` **advisory only**
- [x] Please **merge** this PR for me once it is approved.

.kokoro/lint/common.cfg

@@ -19,7 +19,7 @@ gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
 build_file: "java-docs-samples/.kokoro/trampoline.sh"
 env_vars: {
     key: "TRAMPOLINE_IMAGE"
-    value: "gcr.io/cloud-devrel-kokoro-resources/java8"
+    value: "gcr.io/cloud-devrel-kokoro-resources/java11"
 }
 env_vars: {
     key: "TRAMPOLINE_BUILD_FILE"