ci: switch from testing to bookworm, use ci-deb-repo
With this MR we can use the latest libdrm by updating the PKG_REPO_REV
variable, like Mesa does.
Since ci-deb-repo doesn't seem to support testing, I had to change the Debian image to bookworm (same as Mesa).
The test job I ran earlier shows that libdrm 2.4.122 is used: https://gitlab.freedesktop.org/pepp/xf86-video-amdgpu/-/jobs/60974392#L996