🐹 Migrate to ember 4

[bartocc]

Migrate to ember 4

Ember 3.x deprecations had to be removed to make this possible:

• use [email protected]
• use ember-cli-fastboot@3+
• use a version of guidemaker without ember 3.x deprecations, from Fix implicit-injections and routing.transition-methods deprecations empress/guidemaker#94

The unit test Unit | Service | head data: title with currentPage and currentSelect details is failing because of the ember 3.x deprecation computed-property.override.

I believe that:

1. the test should be re-written with a mock of service:page instead of looking up for it and overriding its currentSection and currentPage CPs
2. the test should belong to https://github.com/empress/guidemaker, not to https://github.com/ember-learn/guides-source, since the head-data service is defined there…

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[bartocc]

Hi there 👋 Anything I can do to help this PR go forward?

[mansona]

Looks great 🎉