Remove debootstrap dependency from riscv64 images

[am11]

This brings riscv64 to the same plan as loongarch64, which uses new mechanism to create rootfs without debootstrap/qemu requirement.

[am11]

@mthalman, with this, we don't need to keep debootstrap up to date any longer, because it's not involved at all.
I tested runtime build locally. Besides, loongarch64 and riscv64 are community platforms using the same build validation pipeline in runtime: https://github.com/dotnet/runtime/blob/8a79637e1454c751c24a984b7fb8af4c4d43394b/eng/pipelines/runtime.yml#L488-L489

[jkotas]

we don't need to keep debootstrap up to date any longer, because it's not involved at all.

Should this be deleted too? https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/1746a09f4ec139b4844413033d7d3cd18614860e/src/azurelinux/3.0/net9.0/crossdeps-builder/amd64/Dockerfile#L32C49-L32C60

[am11]

Debootstrap is still used by officially supported linux architectures. The key is needed for package installation (apt install step) which happens inside chroot environment running on qemu during build-rootfs script: https://github.com/dotnet/arcade/blob/ae3c938af5df4a2db45ded37dd05608ce59d8e5e/eng/common/cross/build-rootfs.sh#L844

@akoeplinger is adding support for signed packages in new (--no-emulation) script dotnet/arcade#15374, so that key will be required there as well. Once signed packages are supported, we can switch official archs to this --no-emulation plan as well and delete things like this script https://github.com/dotnet/dotnet-buildtools-prereqs-docker/blob/1746a09f4ec139b4844413033d7d3cd18614860e/src/ubuntu/build-scripts/install-cross-build-prereqs.sh (which installs qemu in host layer, that is, debootstrap requirement).