downgrade js function vitest dependency to support node 21

[lopert]

Background

Vitest 3.1.3 uses Vite 6+, which in turn does not support node 21.

The CLI currently supports node 21, so our templates need to work on that version.

Solution

Downgrades vitest dep in our function js templates to v2.
Did a find and replace of "vitest": "^3.1.3" -> "vitest": "2.1.9" in all package.json.liquid files.

Tophat

Ran npm install within my function directory with various versions of node and the vitest dep.

Checklist

• I have 🎩'd these changes
• I have squashed my commits into chunks of work with meaningful commit messages