signed-by option should be reflected by installation suggestion

[MrGeneration]

Starting with Debian 11 and Ubuntu 22 apt-key is going to warn administrators about it's deprecation.

Currently this is the suggested way by packager:

wget -qO- https://dl.packager.io/srv/zammad/zammad/key | apt-key add -
wget -O /etc/apt/sources.list.d/zammad.list \
https://dl.packager.io/srv/zammad/zammad/stable/installer/debian/10.repo

However, the repo file looks like this:

deb https://dl.packager.io/srv/deb/zammad/zammad/stable/debian 10 main

It might be good to already include fitting locations etc. In my tests this does work without any issues:

wget -O /tmp/test.gpg https://dl.packager.io/srv/zammad/zammad/key
deb [signed-by=/tmp/test1.gpg] https://dl.packager.io/srv/deb/zammad/zammad/stable/debian 10 main

Of course this doesn't reflect the correct OS paths yet. If it helps I can provide them, I gotta fiddle around for documentation reasons any way.