Bug 33584: Fix failures for Commenter.t on MySQL8

Bug 33584: Fix failures for Commenter.t on MySQL8

The column names from information_schema are uppercase now in MySQL8.
In the arrayref with sliced hashes, we expected lowercase.

See e.g. https://stackoverflow.com/questions/54538448/what-are-the-changes-in-mysql-8-result-rowset-case

Test plan:
Run t/db_dependent/Koha/Database/Commenter.t
Do it again if possible with a MySQL 8 db server.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>