core-deps-latest: Rename quartz-backend to macos-backend for gtk4
It seems that with gtk commit c4d350c260879c11ea94641ac820b331aeaccc7c, the quartz backend option got removed and replaced with the newer macos-backend option.
Meson complains about unknown options, which broke the gtk4 build, so fix that by using the macos-backend build option for gtk when the quartz condition is set.