Bug 34320: Hold reordering arrows look broken after Font Awesome upgrade

Bug 34320: Hold reordering arrows look broken after Font Awesome upgrade

This patch makes minor CSS corrections to the arrows on the place hold
page in the staff interface. The corrections are necessary because of
changes in the display of icons in Font Awesome 6.

To test, apply the patch and locate a bibliographic record in the staff
interface to place a hold on.

- Place at least one hold on that title.
- After placing the hold, you should see a table of all the existing
  holds on that record.
- The arrow control links should look correct, with the "move to top"
  and "move to bottom" arrows having an extra border to differentiate
  them from the others.

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>