chore: upgrade to eslint 9

[ksted]

Why:

Regular dependency update.

Describe your changes

• Upgrades ESLint to v9.
• Migrates configuration to use flat config.
• Updates rules.
• Updates files to comply with the new rules.

Additional references

• Nuxt ESLint
• Vue Storefront ESLint Config
• VueJS Lint Rule Reference

Vsf has a TypeScript configuration as well, but it forces us to use a different Nuxt module configuration that, in combination, crashes the TS parser. The Nuxt module comes with its own set of recommended rules for TypeScript, so I think we're better off using that as base and customising it as we see fit.

Open question: Should we continue to use unicorn rules?

Checklist before requesting a review

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I've updated docs/changelog/changelog_en.md and docs/changelog/changelog_de.md. If I'm a non-German speaker, I've still updated the file with the English version as a placeholder.