chore(ci): track Node 20 deprecation and upgrade GitHub Actions to Node 24-compatible versions

[katriendg]

Issue Description

Context

GitHub has announced the deprecation of Node 20 on Actions runners:
https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

• Node 20 EOL: April 2026
• Node 24 becomes default on runners: June 2, 2026

Work Required

Audit all pinned GitHub Actions in .github/workflows/ and update any that bundle Node 20.
Primary action identified: actions/setup-node (currently pinned to a v4 SHA across 13 workflows).

Acceptance Criteria

• All affected workflows updated to Node 24-compatible action versions
• SHAs remain pinned to immutable release commits (no floating tags)
• $ActionSHAMap in scripts/security/Update-ActionSHAPinning.ps1 updated to include an actions/setup-node@v6 entry pointing to the new v6 release SHA — required so the SHA-pinning update tool stays in sync with the workflows and doesn't emit "not found in ActionSHAMap" warnings on future runs
• npm run lint:version-consistency passes
• CI workflows pass on a test branch before merging

### Additional Context

_No response_