[SSL] Free certificate providers list is outdated

[EtienneM]

https://doc.scalingo.com/platform/app/ssl#generate-your-own-certificate-manually

On this page there is a list of providers which used to sign HTTPS certificates for free: Comodo, InstantSSL and Digicert.

These providers no longer offer free certificates. Cloudflare is a provider which now offers free certificates.