Bug 34379: Fix label of new library group button

Bug 34379: Fix label of new library group button

This patch corrects the label on the button for creating a new library
group, changing it from "Add group" to "New group" for consistency.

To test, apply the patch and go to Administration -> Library groups.
Confirm that the button is now labeled "New group" and that it
still works to trigger the library group creation modal.

Signed-off-by: Andrew <andrew.auld@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 879f7db9e8f835c699ad2a0c8ff9adcb4d7b5d6e)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>