Systemd hook should just use socket

[rjernst]

The systemd hook currently uses jna to call the native sd_notify function. Internally this method just looks for the NOTIFY_SOCKET environment variable that systemd sets, and sends the string arguments to that socket.

We should reimplement our systemd hook to talk to the socket directly. That would simplify the implementation to not rely on jna, and make it easier to move the logic into the server cli once #85758 is merged.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)