[keyserver] Update loader.mjs for new version of Node

Summary: Newer versions of Node replace defaultResolve with nextResolve, and make it return a Promise.

Test Plan: I tested this with both Node 16.9 (.nvmrc) and Node 16.17.1. It doesn't break backwards compatibility with the old version of Node, but it introduces compatibility with the new one

Reviewers: rohan, atul

Reviewed By: rohan

Subscribers: tomek, abosh

Differential Revision: https://phab.comm.dev/D5352