chore: bump helm-diff v3.13.0 → v3.15.7

[Copilot]

Bumps the helm-diff Helm plugin pin to the latest upstream release (v3.15.7).

Description

Upstream version bump: helm-diff v3.13.0 → v3.15.7.

• install.sh — updated tool_version_for case arm for helm-diff
• docs/upstream-versions.md — mirrored new version in the dependency table

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Existing SBOM parser tests (test_parse_install_sh_helm_diff_recorded, test_helm_diff_pinned_from_tool_version_for) already assert v3.15.7 and pass with no changes required.

Test Configuration

• Kubernetes version

Checklist

• My changes follows the style guidelines of this project
• I have performed a self-review of my own changes
• I confirm that a full ./setup/standup.sh -> run.sh -> ./setup/teardown.sh sequence completed successfully
• I confirm that pre-commit run was run and all checks passed
• I have updated the documentation accordingly

[github-actions]

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