This is a package from Debain Bullseye repository. Therefore, I assume you’re are on Bullseye.
This should be unrelated to Kodi version because Kodi is instelled from Debian repository Debian -- Package Search Results -- kodi
Upgrade your system to Debian Bookworm Debian Bookworm has been released – DietPi Blog
Kodi 21 is part of Debian Trixie, the upcoming Debian version which is currently in testing (unrelated to DietPi)
We already offer Trixie images as direct download for testing purposes Index of /downloads/images