Support sha256 hmac for github webhooks

GitHub now also sends a header X-Hub-Signature-256

[ammario]

See #173

Or you can add this to your go.mod to use it before merge:

replace github.com/go-playground/webhooks/v6 => github.com/ammario/webhooks/v6 v6.0.0-20230530210742-61c8d3560b68