Update API Tests for 4.20.x using TestContainers 2.0

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Current API tests only work with 4.17.0.

In 4.20.0 the project is updated to Test Containers 2.0 and Spring Boot 4 (among other). This means the API test needs to be updated/changed so that they work again.

Also the responses of the API endpoints needs to be check if they still work as expected.