Gitlab Webhook 404's

## Issue
gitlab webhook 404's when called from gitlab or if you test the URL directly off of the jenkins server.

### Context
Gitlab plugin version: 1.5.13
Gitlab version: 13.4.3
Jenkins version: 2.249.3
Job Type: Pipeline

### Logs & Traces

Jenkins logs do not provide much information

Nov 19, 2020 9:32:51 AM INFO com.dabsquared.gitlabjenkins.webhook.GitLabWebHook getDynamic
WebHook called with url: /jenkins/project/(removed)

### Problem description

If I try to look at the generated webhook right on the jenkins url, it 404's

URI: | /jenkins/project/(REMOVED)
-- | --
404
Not Found
Stapler

If I test a push event webhook from a gitlab project, I receive a message saying webhook executed successfully but returned 404

These are webhooks and gitlab projects that have previously worked and recently started to fail. I am not entirely sure when they started to fail.

Our Jenkins implementation is behind a F5 load balancer, I tested by bypassing the load balancer and the issue still persisted.