a11y atspi: Send proper dbus reply when (not) changing value

`gtk_accessible_range_default_set_current_value` needs
to return TRUE independent of whether the value was
actually changed, since that return value is required
for the proper dbus reply to be sent to AT-SPI.

Fixes a crash/assertion seen e.g. with the "Hypertext" gtk4-demo
example when trying to change "CurrentValue" for the
level bar via the AT-SPI Value interface:

    GLib-GIO:ERROR:../../../gio/gdbusconnection.c:4354:invoke_set_property_in_idle_cb: assertion failed: (error != NULL)
    Bail out! GLib-GIO:ERROR:../../../gio/gdbusconnection.c:4354:invoke_set_property_in_idle_cb: assertion failed: (error != NULL)
    Aborted

Fixes: #6150

gtk/gtkaccessiblerange.c

@@ -46,7 +46,7 @@ static gboolean
 gtk_accessible_range_default_set_current_value (GtkAccessibleRange *accessible_range,
                                                 double              value)
 {
-  return FALSE;
+  return TRUE;
 }
 
 static void