Change default branch to 3.0.x

Problem/Motivation

The 1.0.x and 2.0.x branches are not used or supported. But 1.0.x is the default value when you go to GitLab.

Change the default branch in GitLab to 3.0.x. Optionally make me a full maintainer so I can do this myself in the future.

Task for @ihmdet or @ifeddi