Complete Node 24 migration: upgrade remaining workflows and engines

[WilliamBerryiii]

Several workflows and packages have been migrated to Node.js 24, but a few remain on Node.js 20. This issue tracks completing the migration.

Workflows to upgrade (node-version: 20 -> 24)

• .github/workflows/docusaurus-tests.yml (line 29)
• .github/workflows/pages-deploy.yml (line 57)

engines.node updates

• docs/docusaurus/package.json (line 6): ">=20" -> ">=24"
• src/500-application/513-tiered-notification-service/package.json (line 14): ">=20.0.0" -> ">=24.0.0"

Optional

• Add .nvmrc at repo root pinning Node 24 for local dev consistency.

Already migrated (for context)

• .github/workflows/cluster-test-terraform.yml
• .github/workflows/code-quality-lint.yml
• .github/workflows/docs-automation.yml
• .github/workflows/docs-lint.yml
• .github/workflows/security-scan.yml