ci: Move Debian from Buster to Bullseye

Now Debian 11 (Bullseye) has been released, shift our CI builds to using
that instead of the older Buster.

Due to dependency-chain changes, we have to install a lot more packages
explicitly and retain more at runtime. This is exacerbated by pkg-config
now requiring the entire chain to be installed, not just the immediate
dependencies.

Our documentation toolchain also gets bumped to a higher version to deal
with Doxygen changes.

Signed-off-by: Daniel Stone <daniels@collabora.com>