Bump org.gitlab4j:gitlab4j-api from 5.3.0 to 5.6.0 (#73)

Author: dependabot[bot]

pom.xml

@@ -14,7 +14,7 @@
     <packaging>hpi</packaging>
 
     <properties>
-        <revision>5.3.0</revision>
+        <revision>5.6.0</revision>
         <changelist>999999-SNAPSHOT</changelist>
         <jenkins.version>2.361.4</jenkins.version>
         <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>