Use signal-cli-jre version by default?

The native version is still experimental and has some issues (signal-cli#3 (closed)).

The java dependencies of the -jre version should be no additional problem, since they are readily available in focal / bullseye repos. The difference in the startup time is not very significant, and scli launches signal-cli child process only once anyway, in the daemon mode.

If a user configures the -native version in update-alternatives, it should be respected, of course. This ticket is only a suggestion to change python3-scli's install dependencies.