Switch default branch to main

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.

Read the discussion in #542, then inspect the repository's branch and release setup. Confirm how changing the default branch affects pull requests and the documented `go install` and `git clone -b releases` flows; done means `main` is the default while the `releases` branch remains usable for older Go versions.

Written by the indexing model from the issue text.