Lock version for yarn and node

[ziyuan-linn]

This PR makes changes related to enforcing dev tool versions, no functional changes.

• Set node.js version to 20.15.1 in package.json and contributing guide.
  • this is currently not enforced in any way
• Set Yarn version to 4.3.1 in package.json and update contributing guide.
  • the correct Yarn version can be installed through corepack
• rebuild yarn.lock for the new Yarn version.

[alanvww]

The content here is not plain text.
Try delete it and run

echo "lts/iron" > .nvmrc

[shiffman]

Looks great, thank you!!