rename `body` to `data` in request options

The `endpoint()` method currently returns `body` as the key holding the request body. We should rename it to `data` to align with `data` parameter in `@octokit/request`: https://github.com/octokit/request.js/#the-data-parameter--set-request-body-directly

We can start by setting both `.data` and `.body` and then logging a deprecation message when `.body` is accessed.