! 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.
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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. I can't find it anymore.
apt-get update apt-get upgrade 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 -y install libgcc-s1 apt full-upgrade libgcc1-