Bug 34677: Update to MARC21 relator terms list (Dec. 14, 2020 technical notice)

Bug 34677: Update to MARC21 relator terms list (Dec. 14, 2020 technical notice)

This patch adds missing relator codes/terms to the marc21_relatorterms.yaml file in accordance with the December 21, 2020 technical notice.

https://www.loc.gov/marc/relators/tn201214src.html

To test:
1. Apply patch and restart
2. Go to Administration > Authorized values > RELTERMS
   --> New term should be there

Addition:
dgc - Degree committee member

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