Autogenerate release messages

[kjagiello]

GitHub just released a public beta of a feature that allows us to automatically generate release notes based on a commit range. Once it goes fully public, it would be neat to support it.

Release post: https://github.blog/2021-10-04-beta-github-releases-improving-release-experience/#introducing-auto-generated-release-notes
Relevant API docs: https://docs.github.com/en/rest/reference/repos#generate-release-notes-content-for-a-release

[kjagiello]

Sample usage of the new endpoint:

#!/usr/bin/env bash

REPO="user/repo"
TARGET_RELEASE=$1
PREVIOUS_RELEASE=$(gh api /repos/${REPO}/releases | jq -r .[0].name)
TARGET_COMMITISH=$(gh api repos/${REPO} | jq -r .default_branch)

gh api \
  /repos/${REPO}/releases/generate-notes \
  -f target_commitish=${TARGET_COMMITISH} \
  -f previous_tag_name=releases/${PREVIOUS_RELEASE} \
  -f tag_name=releases/${TARGET_RELEASE} \
  | jq -r .body