debathena-archive-keyring uses apt-key without depending on gnupg or gnupg2

[andersk]

$ sudo apt install debathena-archive-keyring
[…]
Setting up debathena-archive-keyring (1.5~ubuntu17.04~0.1) ...
Warning: The postinst maintainerscript of the package debathena-archive-keyring
Warning: seems to use apt-key (provided by apt) without depending on gnupg or gnupg2.
Warning: This will BREAK in the future and should be fixed by the package maintainer(s).
Note: Check first if apt-key functionality is needed at all - it probably isn't!
Warning: apt-key should not be used in scripts (called from postinst maintainerscript of the package debathena-archive-keyring)

We should probably fix this by migrating to /etc/apt/trusted.gpg.d (like debian-archive-keyring and ubuntu-keyring did).