Support OpenJDK by default

Oracle stopped providing public updates for Oracle JDK 8 in January 2019. This means that only Oracle customers with a paid subscription or support contract will be eligible for updates.  

In order to prevent implementations from being forced to purchase Oracle JDK licenses, we need to upgrade the platform to run with with AdoptOpenJDK 8. AdoptOpenJDK is free to use, and provides regular maintenance and security updates. Paid support for AdoptOpenJDK is also available from organizations such as IBM. 

The goal of this ticket is for Platform development to use OpenJDK by default while continuing to support Oracle JDK (e.g., CI builds would use OpenJDK and perhaps include a build using Oracle JDK to ensure any incompatibilities with Oracle JDK are caught by CI).