Use python-gitlab package Merge Request Approvals API binding

When the gitlab_adapter.py module was originally implemented the python-gitlab package used did not support the Merge Request Approvals API so curl was used to send the request to the GitLab server. This is now supported in the latest version of the python package.

This MR changes the implementation to use the python-gitlab package Merge Request Approvals API binding to ensure the GitLab API is accessed in a consistent manner for all API calls.