pull archive keyring out of global trust path

The [repository instructions](https://wiki.debian.org/RepositoryInstructions) have been changed to avoid writing third-party keyring files to the global trust anchors (in `/etc/apt/trusted-gpg.d`) and instead write those to a more neutral location (`/usr/share/keyrings`, alongside other keyring files).

The downside of this change is that the key fingerprint isn't validated directly through this process. But considering that validation of the key is anchored through HTTPS validation in the first place, we do not *really* lose anything by moving that to the `.gpg` file transfer: that file's integrity is still checked through HTTPS. Furthermore, not storing the explicit fingerprint here will make future key rotations easier as they will not require documentation updates.

Note that this change will also require a change in the `grml-debian-keyring` package to install the keyring file in the new location. If that package does not install a `.sources` or `.list` file, that move will also break existing configurations, so a NEWS entry might be in order as well.