Backport !1008 “gs-update-monitor: Use wall-clock time for one-notification-per-day check” to gnome-41

Instead of using the g_timeout_add_seconds(), which uses a monotonic time, which may or may not increase when the machine is suspended, rather use the wall-clock time, to avoid issues with machine suspend where the monotonic time does not increase.

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Trivial backport of !1008 (merged) to gnome-41.