Remove some cihper suites.

Marked as weak in SSL Labs' test.

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:!kRSA;
+ssl_ciphers                 CHACHA20:DHE:ECDHE:!NULL:!AES128:!ARIA128:!CAMELLIA128:!SHA1:!kRSA;