Bug 18811: Atomic update to print warning at upgrade time

Bug 18811: Atomic update to print warning at upgrade time

The warning encourages people to run the auth_show_hidden_data script
to check for data in hidden fields and adjust their frameworks.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit f45fc4d10b84dfbd8040d12484553db8c87ec45a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>