Full-upgrade to Debian Testing fails due to libc6-dev : Breaks: libgcc-8-dev (< 8.4.0-2~) but 8.3.0-6 is to be installed

Debian 10 uses GCC 8, but Debian 11 currently uses GCC 9 (probably 10 by the time it’s released). For some reason, in your case, the upgrade fails to replace the GCC 8 packages, and the upgrade is blocked.

To fix this, remove gcc-8 and its dependencies.

This is a symptom of a more general problem with GCC libraries when upgrading from Debian 10 to testing; see Ryan Pavlik’s repository for a general solution and details of the problem, as well as links to bugs filed against GCC in the hope of an official fix.