Upgrade dependencies and setup for test app

[ludovicm67]

This does the following:

• use NPM instead of Yarn
• use new ESLint version
• Upgrade Fastify to v5

[changeset-bot]

⚠️ No Changeset found

Latest commit: 70fb51f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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