Fix broken test in connection.ts for node 18 and add node 18 testing

[JST5000]

High Level Overview of Change

Connection.ts had a broken test with proxy when using node 18.

Context of Change

The error:

  1) Connection
       with proxy:
     Error: Timeout of 20000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves. (/Users/jacksonmills/Documents/xrpl-js-6/xrpl.js/packages/xrpl/test/connection.test.ts)
      at listOnTimeout (node:internal/timers:564:17)
      at processTimers (node:internal/timers:507:7)

Type of Change

• Tests (You added tests for code that already exists, or your new feature included in this PR)

Before / After

Changed localhost to 127.0.0.1 following this workaround: node-fetch/node-fetch#1624

Test Plan

Run tests locally & get a code reviewer to try the same with Node 18

Turn on Node 18 CI