Bump Helm dependencies and CI matrix to v3.21.1 / v4.2.1

[Copilot]

This updates Chartify’s Helm baselines from v3.21.0 / v4.2.0 to v3.21.1 / v4.2.1 to keep module and CI/runtime version targets aligned. The change also refreshes repository docs that define Helm prerequisites.

• Dependency updates

  • go.mod now pins:
    • helm.sh/helm/v3 v3.21.1
    • helm.sh/helm/v4 v4.2.1
  • go.sum is updated accordingly.

• CI version matrix

  • .github/workflows/go.yml now runs against:
    • v3.21.1
    • v4.2.1

• Repository guidance alignment

  • Updated Helm prerequisite/version references in:
    • README.md
    • .github/copilot-instructions.md
    • AGENTS.md

# .github/workflows/go.yml
strategy:
  matrix:
    helm-version:
      - v3.21.1
      - v4.2.1