Bug 29180: Rename syspref RequestOnOpac with OPACHoldRequests

Bug 29180: Rename syspref RequestOnOpac with OPACHoldRequests

It is a confusing name because we have 'Article Request' as a feature,
and this does not follow Koha terminology.

Test plan:
`updatedatabase` and confirm that the syspref has been renamed
`git grep RequestOnOpac` => No occurrence in the code

QA: The syspref's value is always retrieved using Koha.Preference from
.tt files, we don't need to send it from controllers or C4::Auth

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>