[SELKS 6] Loggstash service never stops on shutdown (for reference)

[fdutheil]

Root cause: buggy systemd service unit which doesn't expose ElasticSearch as a dependency for logstash.

Steps to reproduce:

1. first time setup on SELKS 6
2. upgrade via selks script (ELK components get version bumped)
3. on each OS shutdown/reboot attempt: it never happens as logstash nevers stops (wants to send final data into ES but can't as it is already stopped) and waits forever.

Reference for upstream issue with a workaround: https://github.com/elastic/logstash/issues/12344

It would be a clean and persistent workaround for #239 (the one suggested (TimeoutStopSec=180) probably leads to data loss and doesn't survive package upgrade).