chore: update GitHub Actions workflow to streamline Helm chart publis…

[a-klos]

This pull request updates the GitHub Actions workflow for publishing Helm chart packages, simplifying the deployment process and improving how the Helm repo index is managed. The workflow now directly publishes artifacts to the gh-pages branch, removing the previous multi-step deployment to GitHub Pages and associated permissions.

Workflow and deployment simplification:

• Removed unnecessary pages: write and id-token: write permissions from the workflow, reflecting the new deployment approach.
• Replaced the "Build Helm repo index for Pages" step with "Build Helm repo index for gh-pages", adding logic to fetch and merge the existing index from the gh-pages branch if present. This ensures the Helm repo index is correctly updated and avoids overwriting previous entries.

Direct publishing to gh-pages:

• Replaced the upload and deploy steps for GitHub Pages with a single step using peaceiris/actions-gh-pages@v4 to publish the Helm repo directly to the gh-pages branch, simplifying the workflow and eliminating the need for a separate deploy job.