Update Dockerfiles and images for Spack v0.18.0 (#30216)

This PR updates the list of images we build nightly, deprecating 
Ubuntu 16.04 and CentOS 8 and adding Ubuntu 20.04, Ubuntu 22.04
and CentOS Stream. It also removes a lot of duplication by generating
the Dockerfiles during the CI workflow and uploading them as artifacts
for later inspection or reuse.

.github/workflows/generate_spack_yaml_containerize.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+    (echo "spack:" \
+&&   echo "  specs: []" \
+&&   echo "  container:" \
+&&   echo "    format: docker" \
+&&   echo "    images:" \
+&&   echo "      os: \"${SPACK_YAML_OS}\"" \
+&&   echo "      spack:" \
+&&   echo "        ref: ${GITHUB_REF}") > spack.yaml