Add sha512WithRSAEncryption

[amaccuish]

Cipherscan reports my sites don't achieve the "modern" level and tells me to use a certificate signed with sha256WithRSAEncryption. My certs are sign with sha512 which should count.

[tomato42]

for modern level, it does sound ok, but server-side-tls-conf.json is just machine-readable version of settings from https://github.com/mozilla/server-side-tls so this bug should be filed there and referenced from here