Update Node version (20.15 LTS)

[josephjclark]

This PR updates nodejs to 20.15 (the current LTS) and pnpm to 9.5.0

I've set the pnpm version in package.json so hopefully it'll track in future

This LTS build is a year old - here's the release notes

There are no changes that we need to be mindful of.

[josephjclark]

Node 22 is actually stable since April. I might skip onto that once I confirm that 20 works. There are no changes there that look troubling.

[Angeth-Herjok]

worked on this kindly check