Create Pipeline variables are not set properly

[ivancli]

The GitLab API documentation specifies that pipeline variables should be included directly in the request body
(see: https://docs.gitlab.com/api/pipelines/#create-a-new-pipeline).

However, the createPipeline() method in the GitLab PHP Client incorrectly places these variables within a variables field in the request body, leading to a variables is invalid error from the GitLab API.
(see:

Client/src/Api/Projects.php

Line 355 in 362450f

$parameters['variables'] = $variables;

)