Build: Add Felix to .mailmap, update AUTHORS.txt

Beyond the `AUTHORS.txt` changes, this accounts for a recent email change in
one of the commits.

Closes gh-2039

Author: mgol

.mailmap

@@ -34,6 +34,7 @@ Eric Hynds <eric@hynds.net>
 Ethan Romba <ethanromba@gmail.com> <ethan.romba@parivedasolutions.com>
 EungJun Yi <semtlenori@gmail.com>
 Eyal Kobrigo <kobrigo@hotmail.com>
+Felix Nagel <info@felixnagel.com> <fnagel@users.noreply.github.com>
 Filippo Cavallarin <filippo.cavallarin@codseq.it> <poplix@papuasia.org>
 Florian Gutmann <f.gutmann@chronimo.com> <blackfeet@gmx.at>
 Genie <386@mail.com>

AUTHORS.txt

@@ -363,3 +363,5 @@ sakshi87 <53863764+sakshi87@users.noreply.github.com>
 Mikolaj Wolicki <wolicki.mikolaj@gmail.com>
 Patrick McKay <patrick.mckay@vumc.org>
 c-lambert <58025159+c-lambert@users.noreply.github.com>
+Josep Sanz <josepsanzcamp@gmail.com>
+Ben Mullins <benm@umich.edu>