GitLab

* Using Spring Boot Dependencies BOM
* Set up Logback for logging
* Updated dependencies and plugins
* Removed unused dependencies
* Bumped source compatibility to version 1.8

This feature seems to be overkill for the LMS Connector's use case and
has never been used in production.

We remove it because:
* it increased creates code maintenance overhead
* the integrated database increases the the memory footprint

The Ilias connector implementation had an incompatible API and was never
usable with the ARSnova Backend.

:connector-client and :connector-service depend on :connector-model:xjc
task.

statement.

initialization.

header to allowed headers.