Try this
I think that is it this is the output list for my sources.list.d, I assume I would need to update all of these with bullseye:
$ ls -l /etc/apt/sources.list.d/
total 20
-rw-r–r-- 1 root root 60 Feb 17 2019 opensky.list
-rw-r–r-- 1 root root 70 Mar 11 2021 piaware-buster.list
-rw-r–r-- 1 root root 193 Mar 14 2021 raspi.list
-rw-r–r-- 1 root root 38 May 25 2021 rb24.list
I changed raspi.list to https and then had no issues