Adding updated GT ISO3166-2 subdivision codes

[amckenna41]

As of Nov 2021 the ISO has changed the 3166-2 codes for Guatemala (GT) from 2 letters to 2 numbers, for example the code for the department of San Marcos was previously GT-SM but has been updated to GT-12. In the src/pycountry/databases/iso3166-2.json file I've gone ahead and added the updated codes.

Sources: https://www.iso.org/obp/ui/#iso:code:3166:GT https://en.wikipedia.org/wiki/ISO_3166-2:GT