change apt from apt-key to gpg keyring

show available keys

apt-key list

export key and write into file:

gpg --export -a "FusionDirectory Packages Signing Key <contact@fusiondirectory.org>" | gpg --dearmour -o /usr/share/keyrings/fusiondirectory.gpg

change sources.list

vim /etc/apt/sources.list.d/fusiondirectory.list

add [signed-by=] behind deb see:

deb [signed-by=/usr/share/keyrings/fusiondirectory.gpg] https://public.fusiondirectory.org/debian/bullseye-fusiondirectory-release/ bullseye main

and finaly

apt update

without errors