Just download 23H2. The security patches are worth the extra icons in the Start Menu.
To get 21H2, you need the archive or the Volume Licensing Service Center (VLSC) . For consumers, the best bet is the Media Creation Tool for 21H2 . windows 11 iso 21h2 download
However, there is a catch. You should only install this version offline or on machines that will never touch the internet without proper security software. Just download 23H2