Improved documentation for FastBoot users and updated ember-source to v3.20.4

Description

I made 2 changes in this patch release:

• Recommended FastBoot users to update their config/targets.js in order for Node to handle the nullish coalescing operator ?? correctly. (Thanks to @jkeen for feedback!)
• Upgraded ember-source from v3.19.0 to v3.20.4, as well as development dependencies to their latest versions.

The risk in upgrading ember-container-query from v1.1.1 to v1.1.2 should be between none and small.

Notes

For a few weeks, the Ember buildpack for Heroku has had trouble with deploying the demo app (unable to find the latest Node v14.x).

As I don't want to set the Node version in package.json, I may try an alternative service if the problem isn't resolved soon.

Rest assured, this problem does not affect the consuming app.