Adjustments to the Ubuntu ARM repository (mirror.hetzner.com / download.hetzner.com)

Due to an ongoing offset between the Ubuntu upstream repositories of the amd64/i386 (main, archiv) and arm64/armhf (ports) architectures, we are unfortunately no longer able to offer a combined Ubuntu repository as before.

We therefore now offer the ports repository for the Ubuntu arm64/armhf architectures separately and recommend switching to it when running an ARM server. The customization is done via the corresponding file and entry in the `/etc/apt/sources.list.d/` folder or the index file `/etc/apt/sources.list`. The URLs as currently used:

http://mirror.hetzner.com/ubuntu/packages/
http://mirror.hetzner.com/ubuntu/security/

can be replaced by the following:

http://mirror.hetzner.com/ubuntu-ports/packages/
http://mirror.hetzner.com/ubuntu-ports/security/