Set the repository default branch to 3.0.x

Problem/Motivation

When developing merge requests it is nice if the default branch is set.

Steps to reproduce

Proposed resolution

Follow the steps here: https://www.drupal.org/docs/develop/managing-a-drupalorg-theme-module-or... →
Set the default branch to 3.0.x

Remaining tasks

Set the default branch to 3.0.x

User interface changes

API changes

Data model changes