Fix warning suppression for C++20 extension

This adapts a warning suppression so that it works with gcc 12, similar to what was done already in Rec!3230 (merged).

Event/EventPacker/src/component/SelectivePacker.cpp

 /*****************************************************************************\
-* (c) Copyright 2000-2018 CERN for the benefit of the LHCb Collaboration      *
+* (c) Copyright 2000-2023 CERN for the benefit of the LHCb Collaboration      *
 *                                                                             *
 * This software is distributed under the terms of the GNU General Public      *
 * Licence version 3 (GPL Version 3), copied verbatim in the file "COPYING".   *
@@ -838,9 +838,13 @@ namespace LHCb {
         } );
       };
 
-// FIXME: C++20: stop ignoring "-Wpedantic"
+// FIXME: C++20: stop ignoring "-Wpedantic" or "-Wc++20-extensions"
 #pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wpedantic"
+#if __GNUC__ >= 12
+#  pragma GCC diagnostic ignored "-Wc++20-extensions"
+#else
+#  pragma GCC diagnostic ignored "-Wpedantic"
+#endif
       ShoppingList map{*this, requested, m_counters,
                        ShoppingList::Options{.add_track_ancestors  = any_fired( m_addTrackAncestors ),
                                              .add_calo_digits      = any_fired( m_addCaloDigits ),