Change default branch in Gitlab to active branch

Problem/Motivation

Creating a MR or working with anything related to Gitlab defaults to the 6 years idle 7.x-2.x branch.

Steps to reproduce

Visit https://git.drupalcode.org/project/commerce_stock and work with the code. It defaults to comparing to the 7.x-2.x branch.

Proposed resolution

As a maintainer, go to the commerce_stock gitlab:

1. Settings (down the very bottom left)
2. Repository
3. Branch defaults
4. Expand
5. Set the new default branch
6. Save changes