Upgrade yarn to yarn 2

[ManuelMoeri]

Evaluate the upgrade of Yarn Classic(1.x) to Yarn Modern, which is mainly targeted at our Dockerfile still using the old yarn. Make sure everything works as before, even though you might need to update some outdated commands with their new replacement.

Additional context
Migration guide

TODO

• Setup yarn modern for our dockerfile
• Smoke tests
• Update CHANGELOG.md with this improvement
• Update Readme.md if necessary

[ManuelMoeri]

Progress 04.08.2025

The main part of the ticket is basically done, yarn berry has been successfully setup for the prod dockerfile. Yet there are still some thing left do to:

• Check the yarn versions of the other dockerfiles (dev, keycloak) to make sure they also use the new yarn version
• As described above update the changelog (& potentially the readme if the yarn version is mentioned)
• Smoke test to see that everything still works as expected