Pebble should fail CSR requests signed using SHA-1

[aral]

Pebble should fail to finalise a request if it has been signed using the SHA-1 signature algorithm to match:

• The current behaviour of the Let’s Encrypt staging server (as of at least the time of this writing)
• The future behaviour of the Let’s Encrypt production server from September 15, 2022.

(Currently, CSR requests signed with a SHA-1 signature pass under Pebble and fail under Staging.)