Add test for Java 8 Compatibility of third party jars (#1618)

* Add test for Java 8 Compatibility

Add a test to check for Java 8 compatibility of jars deployed to
AppEngine.

It is not enough to run existing tests with Java 8 VM, since many API
jars are not exercised by tests. For example, those for GCP services
like the SecretManager.

We take the conservative approach and verify that every class in every
jar are compiled for Java 8.

build.gradle

@@ -292,7 +292,8 @@ subprojects {
 
   afterEvaluate {
     if (rootProject.enableDependencyLocking.toBoolean()
-        && project.name != 'integration') {
+        && project.name != 'integration'
+        && project.name != 'java8compatibility') {
       // The ':integration' project runs server/schema integration tests using
       // dynamically specified jars with no transitive dependency. Therefore
       // dependency-locking does not make sense. Furthermore, during
@@ -300,6 +301,9 @@ subprojects {
       // immutable. Locking activation would trigger an invalid operation
       // exception.
       //
+      // The ':java8compatibility' project is test-only. Its source does not go
+      // into production.
+      //
       // For all other projects, due to problem with the gradle-license-report
       // plugin, the dependencyLicenseReport configuration must opt out of
       // dependency-locking. See dependency_lic.gradle for the reason why.