Deprecate apt-key operations

Closes #1 . In the future, sources.list files are expected to have a [signed-by=] field, for example:

deb [signed-by=/usr/share/keyrings/i2p-archive-keyring.gpg] https://deb.i2p2.de/ stable main

so:

echo deb [signed-by=/usr/share/keyrings/i2p-archive-keyring.gpg] https://deb.i2p2.de/ $(lsb_release -sc) main | sudo tee /etc/apt/sources.list.d/i2p.list

Will reliably produce a valid sources.list file on modern Debians and Ubuntus.

Existing apt-key users have opted to trust i2p-archive-keyring.gpg to sign packages by using apt-key and this setting will continue to work until the next time the key expires.

If the user does not wish to use a [signed-by=] field, for instance, to build, sign, and install packages locally, the alternative is to copy the key to /etc/apt/trusted.gpg.d instead.