Update default branch in GitLab to 4.x

Problem/Motivation

The GitLab default branch is still set to 8.x-2.x.

Steps to reproduce

Attempt to create a merge request in GitLab. The default target branch is 8.x-2.x.

Proposed resolution

Update the default branch in GitLab to 4.x.

@grasmash I don't have the right permissions to change the default branch in GitLab. Could you make this change?