ci: Switch to upstream ci-templates and use Debian bullseye

This switches to the ci-templates that is found on
https://gitlab.freedesktop.org/freedesktop/ci-templates/



While at it, switch to Debian bullseye, as this contains more reasonably
versioned build tools, i.e. a new enough version of meson.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>