Examine bumping cuvs-java to a minimum of Java 22

[ChrisHegarty]

This GitHub issue has been opened to initiate a discussion about raising the minimum required Java version for cuvs-java.

Currently, the cuvs-java API is compiled against Java 21, while its implementation targets Java 22. The implementation is packaged within the Java 22 section of a multi-release JAR (MR-JAR), meaning that cuvs-java requires a Java 22 (or newer) runtime to function properly. This structure was originally chosen to simplify development, as many Java projects prefer to compile against the latest long-term support (LTS) release — which, at the time of writing, is Java 21.

The main reason for reconsidering the minimum Java version is the use of MemorySegment in the cuvs-java API. Incorporating MemorySegment allows users to transfer datasets between the host and GPU directly from host memory — including memory-mapped files or off-heap memory — instead of relying on on-heap memory or temporary files. This capability can lead to significant performance improvements.

[ChrisHegarty]

Here is a link to a draft PR that demonstrates some of the potential performance benefits we can get from MemorySegment, see #1035, and #1034

[ChrisHegarty]

Huh! I forgot that I came up with an innovative way to avoid this problem before! Since we "just" need MemorySegment, we can model the cuvs-java api through MethodHandles, to avoid the static reference to MemorySegment in the API. I'm going to work with Lorenzo to update his PRs to use this mechanism. When we have this done, and benchmarked, and it works in this context, then we can likely avoid the bump to Java 22 (so, keep the api on Java 21)