deps: upgrade guidemaker deps

[IgnaceMaes]

This PR makes the CLI guides in line with guides-source in terms of setup.

• Upgrades Fastboot and Prember
• Configures ember-showdown-shiki
• Upgrades Node used on Netlify
• Upgrades guidemaker-ember-template
• Align code styling with guides-source

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[MinThaMie]

Discussed during Learning meeting 1st of July;
We would love for it to not have custom css. It looks also good without this :)

[IgnaceMaes]

Reverted the custom CSS for the time being.

The custom styling is currently in guides-source, which is also not the correct place. We should move that to guidemaker-ember-template so it's aligned across all doc sites. But that should be a separate PR to unblock this.

[MinThaMie]

Awesome :) I think we could also move the CSS to style guide if that makes sense :)