chore: replace EOL Node.js v19 w/ LTS v20 in engine spec

[PeterDaveHello]

Update the Node.js engine setting in service/package.json to use LTS v20 instead of the end-of-life v19. This aligns with our Dockerfile that uses node:lts-alpine, which has been a while without issues. The update also helps suppress the runtime warnings about engine compatibility:

WARN  Unsupported engine: wanted: {"node":"^16 || ^18 || ^19"} (current: {"node":"v20.12.2","pnpm":"9.0.4"})