dietpi-software: Icecast: update for native systemd unit on Forky

[MichaIng]

At time of writing, the package has been removed from Debian Forky/testing, since it does not provide a native systemd unit, now mandatory at Debian. A recent update however added a systemd unit, and has been uploaded into Sid/unstable. In a few weeks, it should reach Forky/testing:

• Review native systemd unit of the Debian package and in case adapt it via .service.d override to match our logic. Create own one up to Trixie only.
• Review other packages which do/did not provide an own systemd unit, since it will be the same for all of them.