Change GAE interop tests to use java11 runtime

The Java 8 runtime is end of support. Leaving this a gae-jdk8 for now.
The gae-jdk8 was because AppEngine changed dramatically from Java 7 to
Java 8. Nowadays the versions are more in line with OpenJDK and not very
different from each other.

Fixes #10925

gae-interop-testing/gae-jdk8/src/main/webapp/WEB-INF/appengine-web.xml

@@ -13,8 +13,7 @@
 -->
 <!-- [START config] -->
 <appengine-web-app xmlns='http://appengine.google.com/ns/1.0'>
-  <threadsafe>true</threadsafe>
   <service>java-gae-interop-test</service>
-  <runtime>java8</runtime>
+  <runtime>java11</runtime>
 </appengine-web-app>
 <!-- [END config] -->