Publish helm charts in OCI registry

[janhoy]

We publish helm charts in the classic format at https://nightlies.apache.org/solr/release/helm-charts/

As Helm has now supported OCI for a few years, the old helm repo add mechanism has become less used and represent more steps to do for end user. Also users have reported issues with our current nightlies host, see #812.

I propose the following for our two helm charts

1. Apply INFRA for a new dockkerhub repo "solr-helm"
2. Publish current chart version to OCI
3. Update release process to publish new charts to both HTTPS and OCI
4. Announce to users that we'll discontinue classic charts at end of year, e.g. 2026-12-31
5. Remove https chart repo support

Alternative OCI location could be GHCR, but we already push things to dockerhub and have tokens stored in CI, so it's the easy way.

[janhoy]

I put up a PR as a draft. It uses a manually triggered github workflow for the last publishing step, as the committer then does not need to manage docker hub secrets. Have a look