Rename `master` branch to `main` branch in this repo

[HuangJunye]

What is the expected behavior?

Many (if not all) repos in qiskit github org has renamed master branch to main branch, we should do that in this repo as well. I am not sure about the implications and the steps to do that though.

Related to https://github.com/Qiskit/qiskit/issues/1493

[nandakishore1807]

I think it is not necessary to rename the branchs

[Eric-Arellano]

This repository is being archived very soon in favor of qiskit-terra, so this is no longer relevant.