libmm-glib,common-helpers: undeprecate g_date_time_format_iso8601()

Avoid a deprecation warning with too new glib:

  ../libmm-glib/mm-common-helpers.c: In function ‘date_time_format_iso8601’:
  ../libmm-glib/mm-common-helpers.c:1729:5: warning: ‘g_date_time_format_iso8601’ is deprecated: Not available before 2.62 [-Wdeprecated-declarations]
   1729 |     return g_date_time_format_iso8601 (dt);
        |     ^~~~~~

This call site is protected by version guards and provides an alternative
implementation in date_time_format_iso8601().

libmm-glib/mm-common-helpers.c

@@ -1726,7 +1726,9 @@ static gchar *
 date_time_format_iso8601 (GDateTime *dt)
 {
 #if GLIB_CHECK_VERSION (2, 62, 0)
+    G_GNUC_BEGIN_IGNORE_DEPRECATIONS
     return g_date_time_format_iso8601 (dt);
+    G_GNUC_END_IGNORE_DEPRECATIONS
 #else
     GString          *outstr = NULL;
     g_autofree gchar *main_date = NULL;