Folder issue with gitlab integration

### Version report

[Jenkins](https://www.jenkins.io/) version: 2.303.1
[Gitlab](https://gitlab.com/gitlab-org/gitlab) community edition: 14.5.2
[Jenkins folder plugin](https://plugins.jenkins.io/cloudbees-folder/) version : 6.16
[Jenkins Gitlab plugin](https://plugins.jenkins.io/gitlab-plugin/) version: 1.5.20

Jenkins and plugins versions report generated with

```
println("Jenkins: ${Jenkins.instance.getVersion()}")
println("OS: ${System.getProperty('os.name')} - ${System.getProperty('os.version')}")
println "---"

Jenkins.instance.pluginManager.plugins
.collect()
.sort { it.getShortName() }
.each {
plugin -> println("${plugin.getShortName()}:${plugin.getVersion()}")
}
return
```

output is [here](https://github.com/jenkinsci/gitlab-plugin/files/7725936/output.txt)

- What Operating System are you using (both controller, and any agents involved in the problem)? centos 7

### Reproduction steps

- **Step 1**: Create a Jenkins job in a folder. Use the Jenkins GItlab plugin to get the url that Gitlab should use:


- **Step 2** : Follow the instructions given in the [GitLab documentation](https://docs.gitlab.com/ee/integration/jenkins.html#recommended-jenkins-integration) to input the above in the integration screen


### Results

**Expected result:**

The integration should trigger the jenkins job even if it is located in a Jenkins folder.

**Actual result:**

The integration does not trigger the execution of the Jenkins job. The `Test settings` button in GitLab provides no insights.

**Comment:**

While it is possible to trigger the execution of a Jenkins job through a webhook, the Gitlab documentation presents the integration as the recommended path.

While the webhook and the integration should be equivalent, in terms of report the execution of the Jenkins job to Gitlab, using the integration is presented as a solution [here](https://github.com/jenkinsci/gitlab-plugin/issues/1114).

The [Jenkins Gitlab plugin page](https://plugins.jenkins.io/gitlab-plugin/) asserts that folders are supported.