Update Request Header to Signature-256

[rmtuckerphx]

In March 2022, Amazon changed the header from Signature to Signature-256
See alexa/alexa-skills-kit-sdk-for-nodejs@a165238

This matches the updated docs:
https://developer.amazon.com/en-US/docs/alexa/custom-skills/host-a-custom-skill-as-a-web-service.html#check-request-signature

[mreinstein]

[email protected] uses SHA-256 signature verification now, which should make implementing this pretty trivial (basically just bump the dep and change the request header that is read.)