Ember v3.28.0...v5.2.0

[mixonic]

• Upgrade Ember CLI from 3.28 -> 5.2
• Upgrade Ember source from 3.12 -> 5.2
• Upgrade Ember data from 3.12 -> 4.12

Also

• Adds Volta
• Bumps Node to v16
• Drop Ember versions < 3.28 from the CI
• Add Ember 5 to the CI
• Address many code style issues after the Ember CLI upgrade
• Drop ember-code-snippet, which is old, unmaintained, and not future safe, in favor of a very lame hand-written snippet system.
• Some fixes to the docs pages, which seemed to have gone broken. They are not pretty, but everything works.
• Drop ember-cli-fastboot. It was demanding additional dependencies in order to work correctly which I do not think should be added as library level dependencies. Seems like something to address as/if we split out the docs site (or I will later make an ember-try scenario for the docs site generation if ember-cli-fastboot is a requirement for that, I don't think it is)

[mixonic]

FYI to reviewers, this would start the breaking changes on the road to a v5: https://github.com/html-next/vertical-collection/issues/400