Consider replacing the changelog generator with GitHub's release notes generator

[elia]

It would be a simple API call, using the same token as before, and the contents can be reused for both the changelog and the release.

• Let's also update the docs in case we do the switch https://github.com/solidusio/solidus/wiki/How-to-release-extensions/ac04c9fecff71a5fa895810160e4ebf179c54934.
• See also https://docs.github.com/en/rest/releases/releases?apiVersion=2022-11-28#generate-release-notes-content-for-a-release

curl -X POST \
  -H "Accept: application/vnd.github+json" \
  -H "Authorization: Bearer $CHANGELOG_GITHUB_TOKEN"\
  -H "X-GitHub-Api-Version: 2022-11-28" \
  https://api.github.com/repos/solidusio/solidus_paypal_braintree/releases/generate-notes \
  -d '{"previous_tag_name":"v1.1.1","tag_name":"v1.1.0","target_commitish":"main"}'

cc @waiting-for-dev