Prepare upgrade of xfce CI builder to Ubuntu 26.04

Next Ubuntu LTS is 26.04, and will be the base image for the next Xfce 4.22 (no date yet). Future xfwl4 requires rustc 1.88+, which will be covered by this upgrade too.

Note: Ubuntu 24.04 (our current builder) does have rust 1.89 in backports now.

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 )