libmm-glib,common-helpers: avoid using g_time_zone_new_offset()

It requires newer glib than we do:

  ../libmm-glib/mm-common-helpers.c: In function ‘mm_new_iso8601_time’:
  ../libmm-glib/mm-common-helpers.c:1787:9: warning: ‘g_time_zone_new_offset’ is deprecated: Not available before 2.58 [-Wdeprecated-declarations]
   1787 |         tz = g_time_zone_new_offset (offset_minutes * 60);
        |         ^~
  In file included from /usr/include/glib-2.0/glib/gdatetime.h:33:
  /usr/include/glib-2.0/glib/gtimezone.h:67:25: note: declared here
     67 | GTimeZone *             g_time_zone_new_offset                          (gint32       seconds);
        |                         ^~~~~~~~~~~~~~~~~~~~~~

Let's not use the routine with older versions of glib.

libmm-glib/mm-common-helpers.c

@@ -1783,8 +1783,20 @@ mm_new_iso8601_time (guint    year,
 
     if (have_offset) {
         g_autoptr(GTimeZone) tz = NULL;
-
+#if GLIB_CHECK_VERSION (2, 58, 0)
+        G_GNUC_BEGIN_IGNORE_DEPRECATIONS
         tz = g_time_zone_new_offset (offset_minutes * 60);
+        G_GNUC_END_IGNORE_DEPRECATIONS
+#else
+        g_autofree gchar *identifier = NULL;
+
+        identifier = g_strdup_printf ("%c%02u:%02u:00",
+                                      (offset_minutes >= 0) ? '+' : '-',
+                                      ABS (offset_minutes) / 60,
+                                      ABS (offset_minutes) % 60);
+
+        tz = g_time_zone_new (identifier);
+#endif
         dt = g_date_time_new (tz, year, month, day, hour, minute, second);
     } else
         dt = g_date_time_new_utc (year, month, day, hour, minute, second);