CI: Move from Ubuntu/Focal to Debian/Bullseye for a base image

An important factor of CI is that it can be run on the same environment
that developers typically use.  A problem with Ubuntu/Focal currently is
that due to some trickiness around the host compiler and GCC_VERSION
Kconfig logic, moveconfig.py can no longer sync defconfigs as we run
defconfig without the cross compiler, because we need that defconfig to
be evaluated to know what cross compiler to use.  Since the host
compiler is too old for the arm64 optimized string functions, it is not
asked.  When we syncconfig however, we know the cross compiler, it's now
a "new" option and the syncconfig gets stuck.  While there's no easy
option to provide a more recent toolchain for Ubuntu/Focal,
Debian/Bullseye ships with gcc-10 and we no longer have this problem.

Signed-off-by: Tom Rini <trini@konsulko.com>