monitor-manger: Expose the minimum refresh rate of monitors

This exposes the minimum refresh rate of monitors through D-Bus.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3576>

data/dbus-interfaces/org.gnome.Mutter.DisplayConfig.xml

@@ -359,6 +359,9 @@
 				     it's enabled and second value whether it's
 				     hardware locked (and so can't be changed
 				     via gsettings)
+	    - "min-refresh-rate" (i): minimum refresh rate of monitor when
+	                              Variable Refresh Rate is active (absence
+	                              of this means unknown)
 
         Possible mode flags:
 	  1 : preferred mode