The Debian bullseye-pgdg distribution on apt.postgresql.org has
reached end of life and has been moved to apt-archive.postgresql.org.
There will be no further updates to bullseye. The files on apt.pg.o
will stick around for another 4 weeks and be removed at the end of
September.
See https://apt-archive.postgresql.org for instructions on how to use
the archive if you still have systems using bullseye.
Christoph