Upgrade process may fail inserting records in the `list_table` table, due to the `searchkey_list` value overflowing that column size (`varchar(255)`).
Truncates that field, to ensure upgrade doesn't skip anything.

Wild guess / maybe there's other places we could do this.
Found out a mention of that issue over there: https://listes.renater.fr/sympa/arc/sympa-fr/2019-10/msg00017.html
As of patching that file on my server, I can re-run the upgrade (from 5.4.7 to 6.2.40),  without any mention of such failures.