[buildkite] Update image used in Buildkite JS workflows to Node 20

Summary:
This matches the version we use in Nix, in keyserver/.nvmrc, keyserver/Dockerfile, and services/electron-update-server/Dockerfile.

It should fix a Buildkite CI error I initially saw in the following diff:

error lru-cache@10.2.0: The engine "node" is incompatible with this module. Expected version "14 || >=16.14". Got "16.13.2"
error Found incompatible module.

Depends on D11488

Test Plan: Confirm Buildkite CI passes after rebasing the following diff

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11493