Cause: Underlying rename of libgcc-1 to libgcc-s1. A 'compat' package was provided but removed from debian 11 due to it no longer needing that: gcc was updated to version 9 where this isn't a problem anymore; it'll automatically pull the 's1' version there because gcc-9 only ever has dependencies written using the newer notation.
However, this upgrade is broken on raspberry-pi. The raspberry pi version of debian (raspbian) did not delete/remove gcc-8 so the bug remains there (it's not fixed, but no longer happens to occur, for the normal debian).
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=972820
https://salsa.debian.org/rpavlik/gcc-10-compat
https://packages.debian.org/bullseye/libgcc-s1
https://forums.debian.net/viewtopic.php?t=152931
! Warning !
most advice you see posted about this on the internet is dead wrong! Do not attempt to uninstall libgcc-8. If you do follow this advice, you'll end up breaking your system, because you will end up either without libc (breaks everything, obviously), or crucial tools like awk (depends on libgcc, needed for python build scripts, on which half the system depends). Apt or Apt-get are confused by this name change and will erroneously prefer an older lib when told to update, breaking a bunch of compatibilities.
Apt will try to reason its way out of the tangled mess this creates on some of the lowest level libraries in the system (the C STL). Everything will break, including apt itself, sometimes leaving you with a brick.
The best advice for now is to not update from buster to bullseye at all. Reinstall.
======================
Tenative solution to explore
The following is an experimental method, cobbled together. It'll still break packages, but does not seem to break core systems. It's not a full solution. One exists, but has disappeared from the internet (create a dummy package). I can't find it anymore, because SUSE removed or moved it without telling visitors where it is, and the person that wrote it has not (yet) contacted me back.
The real solution involves (re)creating a proper dummy package somehow. Somehow is big here, debian package format is... complex. I do not have the ability to do so.
sudo bash
# Still breaks things, but hopefully less things.
# Check and re-install (if needed) any non-default software you have on your rPi.
apt-get update
apt-get upgrade
apt-get -y install zstd
sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/raspi.list
apt-get update
apt-get -y install libgcc-s1
apt full-upgrade libgcc1-
apt-mark auto libgcc-s1
apt-mark auto zstd