Update master branch -> main

[phlax]

As im updating envoy's references to master (envoyproxy/envoy#15190) im tracking which related projects have already switched

[chalin]

This repo is a container for markdown files so, IMHO, it should be safe, quick and easy to do the rename since GitHub handles redirects. For details, see:

• github/renaming
• Links to deleted branches now redirect to the default branch

@amye @caniszczyk : would you like me to handle it? (Or maybe @idvoretskyi has the necessary permissions?)

Has someone started to track this change for all the repos under https://github.com/cncf?
Regardless, this repo seems like a simple one to start with :).

cc @celestehorgan @zacharysarah

[idvoretskyi]

@chalin the Kubernetes community is in progress on the switch for multiple repos, and I’m going to track the switching of master->main branches for other CNCF projects after Kubernetes will complete this.

What we’ll do next - add another column to the https://github.com/cncf/toc/blob/main/docs/PROJECTS.csv file, and track the switched projects there.

[idvoretskyi]

@chalin regarding the CNCF org - it’s on my plate, good point!