meta: consolidate AUTHORS information for geirha

Use a .mailmap entry to consolidate the two entries for gierha in
AUTHORS into a single entry.

PR-URL: https://github.com/nodejs/node/pull/40406


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>