Update github to use sha256 signature

[ammario]

See https://docs.github.com/en/webhooks-and-events/webhooks/securing-your-webhooks

[coveralls]

Coverage: 89.741% (+0.02%) from 89.716% when pulling 61c8d35 on ammario:master into 69430a8 on go-playground:master.

[AdamKorcz]

Would be great to see this merged.

[davidhadas]

@robinlieb, this PR relates to a Security Audit done to the CNCF Knative Project,
Knative uses this repository as a dependency and the audit requires this to be fixed.

Can you check to see if this PR is expected to be fixed any time soon, or should Knative look for alternatives instead?

[robinlieb]

@davidhadas just merged this change, thanks for the reminder.

@ammario Thanks for the contribution!