Update helm version to fix installation in newer k8s clusters

What does this MR do?

This MR bumps the helm version to the currently latest in the 2.15 branch (2.15.1), to fix #34164 (closed)

This requires gitlab-org/cluster-integration/helm-install-image!15 (merged)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content