Add support for Helm 4

Summary

Helm 4 has released on 2025-11-12 and features and

represents a significant evolution from v3, introducing breaking changes, new architectural patterns, and enhanced functionality while maintaining backwards compatibility for charts.

While Helm 3 continues to get updates for now, we should look into supporting Helm 4.

Acceptance Criteria

• CI image introduced: GitLab chart: Add Helm 4 CI image (gitlab-org/gitlab-build-images!1038 - merged)
• GitLab chart CI updated to use Helm 4 (maybe with Helm 3 in parallel?)
• Breaking changes addressed
• Tool documentation updated: https://docs.gitlab.com/charts/installation/tools/#helm