Bug 35257: Change 'Circulation desks' to 'Desks'

Bug 35257: Change 'Circulation desks' to 'Desks'

The administration page is the only page that uses "Circulation desks" for
navigation elements. All other areas (sidebar menu, breadcrumbs,
adding and editing desks, page titles) all use just "Desks".

This updates the Koha administration page to use "desks" for consistency. It
also updates the UseCirculationDesks system preference description.

Test plan:
1. Enable the UseCirculationDesks system preference.
2. Go to Koha administration > Patrons and circulation
3. Select the Circulation desks option.
4. Note that the sidebar menu, breadcrumbs, and page titles all
   use just "Desks".
5. Note that in the top-right hand menu bar that it says "NO DESK SET". If
   you click the dropdown menu it says "Set library and desk".
6. Apply the patch.
7. Go to Koha administartion > Patrons and circulation.
8. Note that the heading now says just "Desks".
9. Note that the description for the UseCirculationDesks system
   preference is now "Don't use/Use circulation desks".
10. Sign off D:

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>