Simple… Install Debian Buster (10.x) on your host (or in virtual) instead of Ubuntu 20.04.04 LTS to do your cross compiling… Or even older Ubuntu Bionic…
GLIBC: is always a fun dependency users run into, whenever they use a “system” newer then the “target”…
Ubuntu Bionic (18.04) shipped with GLIBC: 2.27
Debian Buster shipped with GLIBC: 2.28
Ubuntu Focal (20.04) shipped with GLIBC: 2.31
Debian Bullseye shipped with GLIBC: 2.31
This is the way GLIBC has always done it, and every new Ubuntu release, users like you will try to build against and run into problems…
Simply if you want to use our Buster images, either use Ubuntu Bionic or Debian Buster to do your cross building… Otherwise feel free to switch to our Bullseye images, in which case you can use all “4” to cross build…
Regards,