Update to spring-boot 4

[dnlkoch]

Description

This updates to Spring Boot 4.0 and any related library (e.g. Spring MVC, Spring Security and Hibernate).

Note: Since Hibernate is not compatible with Jackson 3, we're still using version 2 here.

This is still a draft! -> Some tests are still failing and not tested and not yet extensively tested.

Related issues or pull requests

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Pull request type

• Bugfix
• Feature
• Dependency updates
• Tests
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe)

Do you introduce a breaking change?

• Yes
• No

Checklist

• I understand and agree that the changes in this PR will be licensed under the
  Apache Licence Version 2.0.
• I have followed the guidelines for contributing.
• The proposed change fits to the content of the code of conduct.
• I have added or updated tests and documentation, and the test suite passes (run mvn test locally).
• I have added a screenshot/screencast to illustrate the visual output of my update.