Set modified flag when loading old s-expr files.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/16113

eeschema/files-io.cpp

@@ -490,6 +490,9 @@ bool SCH_EDIT_FRAME::OpenProjectFiles( const std::vector<wxString>& aFileSet, in
 
             for( SCH_SCREEN* screen = schematic.GetFirst(); screen; screen = schematic.GetNext() )
                 screen->MigrateSimModels();
+
+            // Allow the schematic to be saved to new file format without making any edits.
+            OnModify();
         }
 
         Schematic().ConnectionGraph()->Reset();