Webhook's sent by DoltHub for Pull Request should match GitHub's.

Customer reports that on DoltHub, with "delete source branch" enabled for a repository, they now receive two webhook events, one for a merged pull request being `merged`, and another for a pull request being `closed`.

This is a source of confusion and the distinction between these webhook events isn't clear. If this is unintentional we should aim to match GitHub here and only emit a single webhook event in this case. [Here](https://docs.github.com/en/webhooks/webhook-events-and-payloads#pull_request) is GitHub's documentation.