"wrong shell, use /bin/ksh" warning on OpenBSD

After upgrading to 7.1 recently, I've noticed the following warning in the logs:

```none
starting package daemons:/etc/rc.d/AdGuardHome: wrong shell, use /bin/ksh
```

As far as I know, ksh is mostly compatible with POSIX sh, which is what we usually write our scripts in, so we should investigate, if we should switch.