gitlab-ci: update container to bullseye

The existing buster is oldstable, as such it's stuck with meson 0.49.2 Update it, alongside it we'll get newer clang/clang-format.

Since bullseye features GLVND the whole GL/GLES/EGL/GLX and associated deps have been reshuffled.

Signed-off-by: Emil Velikov emil.velikov@collabora.com