Remove RSA key exchange from the modern SSL config.

SSL Labs' test complains that it doesn't offer forward secrecy.

snippets/ssl-modern.conf

 # vim: ft=nginx
 include                     snippets/ssl-common.conf;
 ssl_protocols               TLSv1.2 TLSv1.3;
-ssl_ciphers                 HIGH:!NULL:!AES128:!ARIA128:!CAMELLIA128:!SHA1;
+ssl_ciphers                 HIGH:!NULL:!AES128:!ARIA128:!CAMELLIA128:!SHA1:!kRSA;