Switch default branch to main

[twifkak]

It is confusing for developers that releases is the default branch. Also a vector for error: sometimes PRs are sent against releases instead.

As of Go 1.16 (I think), we can set semver tags as part of our release process, and go install github.com/ampproject/amppackager@latest will pick that up instead of the default branch. For older versions of Go, users can git clone -b releases and go build ./cmd/amppkg.

Some discussion of (and impetus for) this change at #542.