Change default Branch to Main

### Proposed change

Could we change the default branch to Main from Master?
It has been a couple years

### Alternative options

Some other option where users as a default dont specify a branch

### Who would use this feature?

Anyone using a github.com repo created in the past 2 years

### (Optional): Suggest a solution

I tried to find where it is in the js file but I couldnt figure it out
https://github.com/jupyterhub/nbgitpuller/blob/main/docs/_static/link_gen/link.js

@balajig @sean-morris