Upgrade Java Buildpack

This change updates the submodule for the Java Buildpack from https://github.com/cloudfoundry/cloudfoundry-buildpack-java to https://github.com/cloudfoundry/java-buildpack.  There are many improvements in the new buildpack, and its README.md should be referred to directly for more details.

[#52324703]

**Note:**  In practice, updating a submodule in place is a process rife with problems.  You want to send an email to the team warning them that it'll take some work to get the new submodule looking at the right code.