Remove JDK 9 and 10-based CI builds

[wilkinsona]

Spring Boot 2.1 is currently built and tested against Java 8, 9, 10, and 11. Given that 9 and 10 have been superseded by 11, we should consider dropping the CI builds against Java 9 and 10. We should also decide when we want to start building with Java 12. It's scheduled for release in March 2019 which is before Boot 2.2 will be released. I think it may be worth building 2.1.x against Java 12 so that we're hopefully in a position to support 2.1.x running on Java 12 upon its release.

[wilkinsona]

We're going to drop CI for 9 and 10 and add CI for 12 as proposed above.

[wilkinsona]

Turns out things aren't quite ready in terms of a JDK 12 Docker image. The two choices are either Oracle's JDK on Oracle Linux or OpenJDK on Alpine. Neither uses apt-get for package management so we can't reuse our existing approach for setting up the container.