GitLab CI: update and re-enable it
GitLab CI: review and update it.
Currently the GitLab CI fails because the Meson version provided is not recent enough:
ERROR: Meson version is 0.50.0 but project requires >= 0.51.
From https://gitlab.gnome.org/GNOME/gtk-doc/-/jobs/3642733
The GitLab CI of gtk-doc has not been updated for a while, so an update is required.