Deprecate Java version #57

We previously discussed to only keep supporting one of the VSS service implementations longer-term.

It seems to be more or less consensus that the Rust version is the likelier candidate to keep, especially since ~all other LDK-related codebases are written in Rust.

We should therefore deprecate the Java VSS server, i.e., clearly note the deprecated status in the README (so new users don't start off on Java, sparing them the later migration), and possibly considering archiving the Java codebase at a later date.