fix(connection): unref the "_waitForConnect"

[hasezoey]

Summary

The recent change in #15229 has added a timeout, which can lead to having the node process around for longer than expected (ie until the timeout has fired), which can cause jest to detect open handles.
(I had also encountered this while upgrading typegoose)

Also the check of unref existing is made for alternative engines like deno and bun, where i had previously encountered that unref may not always be implemented.

fixes #15241

[vkarpov15]

I'm not entirely convinced this is the right behavior because unref() here undermines the primary purpose of _waitForConnect(), which is to error out if the user tries to execute operations on a connection that is never opened. For example, with this change, the following script would exit immediately with no output:

'use strict';

const mongoose = require('mongoose');

(async function() {
  await mongoose.connection.listCollections();
})();

I'm not saying we shouldn't merge this, but I would like to discuss further in #15241 to understand the use case here.

[hasezoey]

This PR can be closed once #15251 lands, or should this be kept open for something?