CI: Use Debian 11 'bullseye' for most builds

CI: Use Debian 11 'bullseye' for most builds

dbus 1.12.x was included in both Debian 10 'buster', which is now EOL,
and Debian 11 'bullseye', which continues to be supported. I need this
branch to work on Debian 11 for security and bugfix backports, but I'm
no longer supporting Debian 10.

Leave one build variant, the "legacy" build, on Debian 10 to check that
we can still build there, to be nice to the Debian LTS subproject.

Also continue to use Debian 10 for mingw builds, since dbus#380 has
not been fixed in this branch (which is now the security-fix-only
old-stable branch, so the fix will not be backported).

Signed-off-by: Simon McVittie <smcv@collabora.com>