Renaming base branch from "master" to "main"

[naveen521kk]

This is to announce that the "master" branch would be renamed to "main" on 2021-06-02 (after the release of Manim v0.7.0).

There had been some discussion (private) on doing this on Discord and a poll was conducted where many community devs decided on selecting main as the default branch.

Prepare your scripts that would refer to "master" for the change.

Rationale:

• https://github.com/github/renaming
• https://sfconservancy.org/news/2020/jun/23/gitbranchname/