Bug 31653: Correct event names passed to jQuery on() function

Bug 31653: Correct event names passed to jQuery on() function

This patch replaces the obsolete .on("hover") with .on("mouseenter
mouseleave"). This will allow the code to work again following the last
jQuery upgrade.

See Bug 20217 for discussion of the original implementation:
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20217#c14

To test, apply the patch and go to the staff interface. Hover your mouse
over the "Search" link in the header menu. The adjacent dropdown menu
button should have its hover state triggered.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>