Prepare upgrade of xfce CI builder to Ubuntu 26.04

Next Ubuntu LTS is 26.04, and may be the base image for the next Xfce 4.22 (no date yet). Future xfwl4 requires rustc 1.88+, which can then be added the image (26.04 includes rustc 1.93).

Note: Ubuntu 24.04 (our current builder) does have rustc up to 1.91

For 4.20 branches, we can keep the xfce-build:24.04 (by including ci/build_project.yml from xdt 4.20 in 4.20 .gitlab-ci.yml branches / https://docs.gitlab.com/ci/yaml/#includeproject ). We need to push an image tagged 24.04 to the registry.

Note²: Debian 13 has rustc 1.91 in backports, so this is recent enough too.