Build of Pivio is broken due to failing tests

[obfischer]

As maintainer of Pivio server, I want every build to be successful and all tests to run successfully so that the development of Pivio can continue.

Some of the used frameworks and libraries are now available in newer versions, therefore these dependencies should also updated. This will help to improve the test architecture of Pivio server.

Therefore the issue consists of the followin subtasks:

• [x] Update ElasticSearch to major version 6
• [x] Update SpringBoot to major version 2
• [x] Upgrade Gradle to version 5 or 6
• [ ] Replace the deprecated Delete-By-Query-Plugin in favor of the Delete by Query API
• [ ] Run the integration tests as Spring Boot tests
• [ ] Upgrade JUnit to version 5
• [ ] Ensure that Pivio has the same functionality as before

[obfischer]

Work will be done on the branch modernization.