Renaming the dafault branch

[qinkunbao]

To follow the inclusive language best practice, we should rename the default branch to main.

URL:
https://github.com/github/renaming

[mssun]

Yes, to do this, we need the help of the INFRA team. Specifically, we can submit an issue to the INFRA team [1].

• https://issues.apache.org/jira/projects/INFRA/issues