Why Filebeat did not restart after apt upgrade?

## Issue ##

Why doesn't Filebeat restart after an upgrade (apt upgrade)?
It runs the previous version until a systemctl restart filebeat is executed after the upgrade.

I looked at the `postinst` in the debian package and there is no code to restart the service. Does it have a reason?

```
#!/usr/bin/env bash

systemctl daemon-reload 2> /dev/null
exit 0
```

This issue is not only happening on Filebeat but on every Beat.

## Definition of done ##

- [ ] Beats are correctly restarted after an upgrade
- [ ] Testing in place to validate that new version is running after an upgrade

Start by examining the Debian package postinst shown in the issue and the systemctl daemon-reload step. Reproduce an apt upgrade followed by systemctl status for Filebeat or another Beat, then determine how the package upgrade should ensure the new version is running. Done means Beats restart correctly after upgrades and testing validates that the new version is active.

Written by the indexing model from the issue text.