[chore] Upgrade to node 20

[yoannmoinet]

Summary

This PR upgrades the project from Node.js 18 to Node.js 20, updating all related dependencies and simplifying our CI configuration.

Changes

Runtime & Dependencies

• Updated Node.js from 18.20.5 to 20.19.4 (via Volta configuration)
• Updated all @types/node dependencies from ^18 to ^20 across the monorepo

Code Updates

• Removed TypeScript error suppressions for fs.openAsBlob in packages/core/src/helpers/fs.ts
  • This API is now properly typed in Node.js 20
• Use with instead of assert when importing JSON files.

CI Simplification

• Removed Node.js version matrix from GitHub Actions workflow, CI now tests only against the version specified in package.json
• This reduces CI complexity and execution time