Bump node version for setup,sign and verify action to 24

[sridharam]

https://endorlabs.atlassian.net/browse/CR-750

Bump sub-action Node.js runtime from node16 to node24

Upgrades the Setup, Sign, and Verify sub-actions from the deprecated Node.js 16 runtime to Node.js 24, aligning them with the main Scan action which was upgraded in #90.

Changes

• setup/action.yml → using: "node24" (was node16)
• sign/action.yml → using: "node24" (was node16)
• verify/action.yml → using: "node24" (was node16)

Context

• Node 16 reached EOL in September 2023.
• GitHub fully removed Node 16 from Actions runners on November 12, 2024. Actions declaring using: "node16" have been silently falling back to Node 20 since then — the YAML was misleading but not causing failures.
• The main Scan action (action.yml) was upgraded to node24 in Bump node versions to Node 24 #90, but the sub-actions were missed.
• No source code or dist/ changes required — the using field only controls which Node.js runtime GitHub uses to execute the bundled JS on the runner. The bundled code targets ES2016 and uses only standard Node APIs compatible across all versions.

Testing

• All CI checks passing (build, lint, format, tests, license)
• End-to-end validation on the branch:
  • Setup action: passed — endorctl download, config write, and auth check all working on node24
  • Sign action: passed — action starts and runs correctly on node24
  • Verify action: passed — action starts and runs correctly on node24