Upgrade rails to 6

[matthijsgroen]

What

Explain what changes inside the code, this can be as simple or complicated as you want as long as it's clear

Why

Explain why these things are changes. This explanation is for your colleagues and your future self.

Code Review

Please consider the following checklist when reviewing this Pull Request.
More background and details here.

• Does the code actually solve the problem it was meant to solve?
• Is the code covered by unit tests? Integration tests?
• Does anything here need documentation? (Focus on why, not what.)
• Does any of this code deal with privacy sensitive information or affects security? Ask an additional reviewer.
• Is the code easy to understand and change in the future?
• Is the same code or concept duplicated? Find a balance between DRYness and readability.
• Does the code reasonably adhere to the Kabisa coding standards?
• Be kind.

[matthijsgroen]

When running locally, all tests are now green:

docker compose up --build
docker exec -it kudo-o-matic_web bash
RAILS_ENV=test bundle exec rspec

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