debian package: apt-key is deprecated; new installation commands need to be documented

[skyper]

The command to download and store the gpg-key for the ubuntu package need to be updated under Download as apt-key add is deprecated as insecure, see ​https://manpages.debian.org/testing/apt/apt-key.8.en.html.

According to ​https://wiki.debian.org/DebianRepository/UseThirdParty?action=show&redirect=RepositoryInstructions#OpenPGP_Key_distribution the key should not be ascii-amored and stored in a separate directory beside /etc/apt/trusted.gpg.d/, e.g. /usr/local/share/keyring/, which might not exist so far, see ​https://wiki.debian.org/DebianRepository/UseThirdParty?action=show&redirect=RepositoryInstructions#Sources.list_entry

As this change only changed the commands but not the underlying security issue, I reverted it.

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• The command to remove the key from /etc/apt/trusted.gpg and /etc/apt/trusted.gpg.d/ is something like

sudo apt-key del $(apt-key finger | grep -B1  "<josm-dev@openstreetmap.org>"| sed '$d' | cut -b 48-51,53-57)