Simplify GTK+ 2 version checks with a helper function

Use wx_is_at_least_gtk2(minor) instead of gtk_check_version(2, minor)
because it is more clear and also works as expected (i.e. returns true)
when using GTK+ 3 instead of creating difficult to notice bugs in the
code by failing the version check in this case.

See #18004.

src/unix/utilsx11.cpp

@@ -2680,9 +2680,7 @@ wxDoLaunchDefaultBrowser(const wxLaunchBrowserParams& params)
 {
 #ifdef __WXGTK__
 #if GTK_CHECK_VERSION(2,14,0)
-#ifndef __WXGTK3__
-    if (gtk_check_version(2,14,0) == NULL)
-#endif
+    if (wx_is_at_least_gtk2(14))
     {
         GdkScreen* screen = gdk_window_get_screen(wxGetTopLevelGDK());
         if (gtk_show_uri(screen, params.url.utf8_str(), GDK_CURRENT_TIME, NULL))