Fix lib-core reference (related to 9de20f59)

SonarQube analysis indicates that quality gate is passed. * Reliability Rating on New Code is passed: Actual value 1 * Security Rating on New Code is passed: Actual value 1 * Maintainability Rating on New Code is passed: Actual value 1 * Coverage on New Code is passed: Actual value 0.0 * Duplicated Lines (%) on New Code is passed: Actual value 0.0 * Security Hotspots Reviewed on New Code is passed: Actual value 0.0 SonarQube analysis reported 2 issues * :arrow_down_small: 1 minor * :information_source: 1 info Note: The following issues were found on lines that were not modified in the commit. Because these issues can't be reported as line comments, they are summarized here: 1. :arrow_down_small: [This assertion is unnecessary since it does not change the type of the expression.](https://gitlab.hochschule-stralsund.de/laboration/operator-environment/blob/cee2a9d9f87c6eb480cb4a238930e3a6059a1d41/src/operator/creator.service.ts#L236) [:blue_book:](https://sonarqube.lui-studio.net/coding_rules#rule_key=typescript%3AS4325) 1. :information_source: [Complete the task associated to this "TODO" comment.](https://gitlab.hochschule-stralsund.de/laboration/operator-environment/blob/cee2a9d9f87c6eb480cb4a238930e3a6059a1d41/src/operator/deleter.service.ts#L65) [:blue_book:](https://sonarqube.lui-studio.net/coding_rules#rule_key=typescript%3AS1135)