Add GitHub Admin Team members voted in via mailing list[1]

[1] https://groups.google.com/g/kcp-dev/c/ZE12NB-MDPA/m/l9t-IDy1AQAJ

Signed-off-by: Marvin Beckers <[email protected]>

Author: embik

MAINTAINERS.md

@@ -28,4 +28,7 @@ The following maintainers are members of the security response team and enact th
 
 The following maintainers are members of the GitHub Admin Team and manage the GitHub organizations:
 
-- TBD
+- Dr. Stefan Schimanski
+- Mangirdas Judeikis
+- Sebastian Scheele
+- Christoph Mewes