Downgrades the multiarch build environment from Ubuntu 24.04 to 20.04.
This ensures that native artifacts for aarch64 and ppc64le are linked
against glibc 2.31, providing better compatibility with older but
still-supported distributions (like RHEL 8 and Ubuntu 20.04).

Additionally, this consolidates s390x cross-compilation into the
multiarch image, removing the need for a separate ubuntu2004 base
image.

- Update Dockerfile.multiarch.base to use ubuntu:20.04
- Add g++-s390x-linux-gnu to multiarch image
- Update linux_artifacts.sh to use multiarch for all cross-builds
- Remove references to Dockerfile.ubuntu2004.base in build scripts

  Summary of what this commit achieves:
   1. Improves Compatibility: Lowering the glibc requirement from 2.39 (Ubuntu 24.04) to 2.31 (Ubuntu 20.04) prevents "version `GLIBC_2.XX' not found" errors when users try to run the artifacts on older
      systems.
   2. Simplifies CI: Merges the s390x toolchain into the multiarch image, reducing the number of Docker images the project needs to build and maintain.
   3. Addresses Reviewer Feedback: Directly implements the "choice" of Ubuntu 20.04 suggested by the reviewer as a safer stopgap for the build.

Fixes grpc#12797.