meta: align collaborator name in .mailmap/AUTHORS with README

I exchanged email with Gerhard to make sure that this was the rendering
of his name that was correct.

PR-URL: #39489
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

.mailmap

@@ -146,7 +146,7 @@ Gabriel de Perthuis <g2p.code@gmail.com>
 Gareth Ellis <gareth.ellis@uk.ibm.com> <gareth@gsellis.com>
 Garwah Lam <garwahlam@gmail.com>
 Geoffrey Bugaisky <gbugaisk@gmail.com>
-Gerhard Stoebich <deb2001-github@yahoo.de>
+Gerhard Stöbich <deb2001-github@yahoo.de>
 Gibson Fahnestock <gibfahn@gmail.com> <gib@uk.ibm.com>
 Gil Pedersen <git@gpost.dk> <github@gpost.dk>
 Graham Fairweather <xotic750@gmail.com> Xotic750 <xotic750@gmail>

AUTHORS

@@ -1489,7 +1489,7 @@ Madara Uchiha <me@madara.ninja>
 Gil Tayar <gil@tayar.org>
 Glenn Schlereth <Glenn.Schlereth@gmail.com>
 Artur G Vieira <vieira.artur.g@gmail.com>
-Gerhard Stoebich <deb2001-github@yahoo.de>
+Gerhard Stöbich <deb2001-github@yahoo.de>
 Sreepurna Jasti <sreepurna.jasti@gmail.com>
 Rafael Fragoso <rafaelfragosom@gmail.com>
 Andrei Cioromila <andrei.cioromila@gmail.com>