Migrate upstream Helm chart source to blob.istio.io

General MR

Summary

This fix addresses Istio retiring its GCP-hosted release infrastructure by December 2026: https://istio.io/latest/blog/2026/retirement-of-gcp/#helm-charts-and-other-release-artifacts

Migrates the upstream Helm chart source off the retiring Istio GCP endpoint (istio-release.storage.googleapis.com) to https://blob.istio.io/istio-release/charts.

Deployed clusters are unaffected — the istiod-1.30.4.tgz pulled from the new endpoint is byte-identical to the vendored chart, so chart/charts/ is unchanged.

Relevant logs/screenshots

helm dependency build ./chart
# Downloading istiod from repo https://blob.istio.io/istio-release/charts
# succeeds from a clean checkout; vendored tarballs unchanged
grep -r istio-release.storage.googleapis.com .
# no matches

Linked Issue

#85 (closed)

Upgrade Notices

N/A