Store ArtifactHub metadata by version

[AkashKumar7902]

Summary

• write each released policy ArtifactHub metadata under <policy>/<version>/
• copy the release README into the same version directory
• remove the old flat artifacthub-pkg.yml and README.md files during migration so Artifact Hub indexes the multi-version layout

Fixes #160

Testing

• yq e . .github/workflows/release.yml >/dev/null
• extracted the edited push-artifacthub shell block and ran bash -n after replacing GitHub expressions
• git diff --check

Reference: Artifact Hub documents multi-version Kubewarden policy packages as <package>/<version>/{README.md,artifacthub-pkg.yml}.

[viccuad]

This is a change with the potential to cause huge disruptions.

I would like to see a test where a repo with these changes is used in https://staging.artifacthub.io/ as source prior to merging.

[AkashKumar7902]

Agreed. This changes the layout Artifact Hub indexes, so a staging import is the right gate before merging.

I cannot honestly complete that staging.artifacthub.io validation from this environment because it needs a repo/source registered in Artifact Hub staging. I will keep this PR blocked until we have that test result. Once the staging source is configured and indexed, I will update the PR with the result and any changes needed from the import logs.