Switch to using OpenAPI spec for webhooks

[wolfy1339]

Hello,

This repository was linked over in octokit/webhooks#911, and I noticed you are still using the community generated schemas.

The @octokit/webhooks-schemas package will be deprecated in the near future and will no longer receive updates, see octokit/webhooks#901

I recommend you switch over to https://github.com/octokit/openapi-webhooks

https://unpkg.com/@octokit/openapi-webhooks/generated/api.github.com.json

Unfortunately, this means that you will need to refactor your code, and is probably best bundled in a breaking change.

[Miaxos]

Thank you for your input! I'll update it accordingly soon so :-)