build: downgrade multiarch to Ubuntu 20.04 and consolidate images (1.82.x backport)

[ejona86]

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. Implements stopgap: Directly implements the choice of Ubuntu 20.04 suggested as a safer stopgap for the build.

Fixes #12797.

Backport of #12830