Bug 35524: Fix terminology in basket group CSV export

Bug 35524: Fix terminology in basket group CSV export

Bookseller should be vendor to match our preferred terminology.

To test:
* Run:
  prove t/db_dependent/Acquisition/GetBasketGroupAsCSV.t
* Add a basket with at least one order line
* Close basket and add to a basket group
* Close the basket group
* Export as CSV
* Verify headers include "Bookseller"
* Apply patch, restart all
* Repeat export, it should now say "Vendor"
* Rerun tests

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>