code: public Milestone closeGroupMilestone(Object groupIdOrPath, Integer milestoneId) throws GitLabApiException {
  
          if (milestoneId == null) {
              throw new RuntimeException("milestoneId cannot be null");
          }
  
          GitLabApiForm formData = new GitLabApiForm().withParam("state_event", MilestoneState.CLOSE);
          Response response = put(Response.Status.OK, formData.asMap(),
                  "groups", getGroupIdOrPath(groupIdOrPath), "milestones", milestoneId);
          return (response.readEntity(Milestone.class));
      }
docs: Close a group milestone.
  
  @param groupIdOrPath the group in the form of an Integer(ID), String(path), or Group instance
  @param milestoneId the milestone ID to close
  @return the closed Milestone instance
  @throws GitLabApiException if any exception occurs