main branch should be numeric, not 'main'

Problem/Motivation

This project's main branch is called 'main', but Drupal's git integration expects numeric branch names, e.g. 1.x or 1.0.x.

One of the problems this causes is that creating an issue fork doesn't offer to create the issue branch too.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes