Update to Ember v4.12

[Windvis]

This updates the project to the latest Ember v4.12 blueprint and resolves any new linting errors.

I tried resolving all deprecations I encountered, but some of them are in addon code.

Remaining deprecations:

• ember-data-fastboot triggers the toArray deprecation here. We could maybe patch it? Or inline the code in the repo? The package seems pretty unmaintained. I'm also not really sure we actually need it? Does it provide a huge benefit over fetching the data again in the frontend?
• ember-leaflet triggers a ember-render-helpers deprecation, but that shouldn''t block the v5 blueprint update in the future.

Supersedes #1076

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[Windvis]

I created a follow-up PR that simply removes ember-data-fastboot: #1194

That seems like the easiest and imo best solution. I don't think loading an extra json on the initial page load is that bad and it allows us to drop an addon that is not maintained, or having to maintain that code here.

Since the end result of this repo is most likely a Vite + SSR setup it would have to be dropped eventually anyway.

[mansona]

I have simplified this PR by splitting out the lint fixes into their own PR #1197 to make it easier for me to review 👍