Migrate to main branch on GitHub

[bcoe]

We're in the process of migrating to using main as the primary branch name on GitHub.

It would be good if gax-nodejs was migrated, so that it's consistent with other Node.js repositories.

You will need to be mindful of any automation configured on this repository.

[summer-ji-eng]

Try to clarify that are you propose to use main branch to replace master?
Any timeline expect to delivery?

[summer-ji-eng]

We did the migration to main. Close for now.