The
https://github.com/dotnet/runtime/blob/main/eng/common/cross/build-android-rootfs.sh

Go line: 100

Find string:
http://termux.net/
Replace all to:
https://packages.termux.dev/termux-main-21/

Build succeeded test:
./build.sh --os Android
or
ROOTFS_DIR=$(realpath /Home/runtime_linux/.tools/android-rootfs/android-ndk-r21/sysroot) ./build.sh --cross --arch arm64 --subset mono

@steveisok