F null errors

Summary

Fix problems raised by @i.nueske while working on https://gitlab.indiscale.com/caosdb/customers/gwdg/management/-/issues/149

• client stubs for repositories and profiles endpoints could not be generated
• listRepositories returned with null-error
• update spring-boot version on the go

Focus

• Remove the nullable keyword: The python client generator doesn't support the "nullable"-keyword in parameter even though the specs do not restrict it that way, see https://swagger.io/docs/specification/describing-parameters/#nullable However, we don't need the nullable keyword here, as the spring-boot generator doesn't support the nullable keys at all - seems to be a pretty new feature without widespread use.
• null-valued type: we just forgot to set it.
• everything else are minor adjustments or unrelated stuff

Testing

Please test with generated python client code.

Check List for the Author

• All automated tests pass
• Reference related issues
• Appropriate user and developer documentation for new code
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Appropriate user and developer documentation for new code
• The test environment setup works and the intended behavior is reproducible in the test environment

For further good practices have a look at our review guidelines.