fix: pin gradle/maven extraction to jdk8 instead of using gcp provided image (#3524)

This way when [gcp's maven builder image](https://cloud.google.com/cloud-build/docs/cloud-builders) changes its default version, we won't get unpredictable behavior, because we'll be pinned to a specific version from https://hub.docker.com/_/maven.

Also some fixes to gradle examples to bring them more in-line with maven examples.
12 files changed