EPIC: Automate the update external test material versions

We have been doing this manually, changing the tag_version values in set_test_info in [common_functions.sh](https://github.com/AdoptOpenJDK/openjdk-tests/blob/master/external/common_functions.sh#L176), but should also look at what it would take to automate this.

Related:
https://developer.github.com/v3/repos/releases/#get-the-latest-release
https://stackoverflow.com/questions/58465057/trigger-a-github-action-when-another-repository-creates-a-new-release

Where possible, we want to automate the update of external test material to latest stable versions. In some cases, where the tagging and releases of external test material is 'not predictable' we will still have to manually manage, but in cases where we can automate we should. Additional caveat: some external test material can be used for certain versions of Java, and we may need to support different tags/releases depending on which JDK_VERSION we are testing.