Align GLib dependency requirements

It looks like https://gitlab.freedesktop.org/pwithnall/malcontent/blob/master/libmalcontent/tests/meson.build requires GLib 2.60 when the rest of the project requires glib-2.0 ≥ 2.54 (as specified in the README.md) it would probably be better to align the whole project with the same requirements.