Rename the `jupyter/notebook` repo default branch to `main`

[jtpio]

As part of jupyter/notebook#6266, we should consider renaming the default branch of the https://github.com/jupyter/notebook/ repo to main.

Many Jupyter subprojects have already done it:

• https://github.com/jupyter-server/jupyter_server
• https://github.com/jupyterlab/lumino
• https://github.com/jupyterhub/jupyterhub/

And GitHub now defaults to main when creating a new repo.

This can be done automatically via the GitHub UI:

[jtpio]

cc @Zsailer @kevin-bates since you have been maintaining the repo over the past months

Let me know if you have any concerns about the branch renaming. Otherwise I would suggest we do it this week.

[kevin-bates]

This sounds fine to me - thank you Jeremy!