Update deprecated nginx `http2` directive

Seen this on beta while building with SSL enabled:
```
[warn] 18#18: the "listen ... http2" directive is deprecated, use the "http2" directive instead in /etc/nginx/conf.d/default.conf:22
nginx: [warn] the "listen ... http2" directive is deprecated, use the "http2" directive instead in /etc/nginx/conf.d/default.conf:22
[notice] 18#18: signal process started
```
