Gtilab MR*s webhook trigger jenkins using only master branch for pulling code

### Jenkins and plugins versions report

When creating or updating a merge request, the webhook triggers the pipeline, but instead of using the source branch (the one with the changes), it always takes the master branch.


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

Gitlab plugin version: 1.5.20
Gitlab version: 15.8.5-ee
Jenkins version: 2.263.4
Job type: Pipeline

### Reproduction steps

Create or update a merge request.
Job start with master branch.
Pipeline runs with wrong configuration(outdated from master not my new branch).

### Expected Results

Job start with _source_ branch(Branch where the changes were made).

### Actual Results

Pipeline runs with wrong configuration(outdated, from master not my new branch).

### Anything else?

_No response_