Update GitHub signature header to use sha256

[nlecoy]

Hi !

First of all, thanks for the work done on Drone.

Just a quick PR to update the header used to validate webhooks from GitHub.
https://docs.github.com/en/developers/webhooks-and-events/webhooks/securing-your-webhooks#validating-payloads-from-github

GitHub still supports X-Hub-Signature for backward-compatibility but recommend using the X-Hub-Signature-256

[CLAassistant]

All committers have signed the CLA.

[tphoney]

Hey @nlecoy thanks for raising this up, unfortunately using a better encryption is normally the way to go. This library needs to always work, and with this change we could potentially break older installations of github server.
IE we will lean here towards compatibility rather than security here.

Though we are open to further discussion.

[nlecoy]

Hi there,

Sorry to comme back to you just now (didn't see first answer). I added your suggestion @bradrydzewski and added some test.

[tphoney]

Hey @nlecoy thanks for the PR then the additional work. !