Add a mailmap entry for y21

Author: tgross35

.mailmap

@@ -652,6 +652,7 @@ Tim Hutt <tdhutt@gmail.com>
 Tim JIANG <p90eri@gmail.com>
 Tim Joseph Dumol <tim@timdumol.com>
 Tim Neumann <mail@timnn.me> <timnn@google.com>
+y21 <30553356+y21@users.noreply.github.com>
 Timothy Maloney <tmaloney@pdx.edu>
 Tomas Koutsky <tomas@stepnivlk.net>
 Tomasz Miąsko <tomasz.miasko@gmail.com>