Jib build template - differentiate Java 8 vs Java 11

[saturnism]

Some Java 8 applications can't be built/run w/ OpenJDK 11. Jib build template uses the maven base image: gcr.io/cloud-builders/mvn which isn't pinned to a JDK version. As a result, the latest version of this image jumped from OpenJDK 8 to OpenJDK 11, resulting in build failures.

We should let user choose the base JDK version, potentially through:
a. different build template name (e.g., jib-maven-jdk-8) and using a pinned maven base image
b. have both OpenJDK 8 and 11 in the same base image, and let user configure via an environmental variable to specify which OpenJDK version to use.

/cc @mchmarny @coollog