Remove CombinatorialSpecSet in favor of environments + stacks

etc/spack/defaults/release.yaml

lib/spack/docs/example_files/spec_set.yaml

lib/spack/spack/cmd/buildcache.py

lib/spack/spack/cmd/release_jobs.py

lib/spack/spack/schema/cdash.py

lib/spack/spack/schema/gitlab_ci.py

lib/spack/spack/schema/spec_set.py

lib/spack/spack/spec_set.py

lib/spack/spack/test/cmd/release_jobs.py

lib/spack/spack/test/spec_set.py