add OCSP stapling to nginx.conf

It's not added in tls.conf, because apparently the mail ssl module
doesnt' support OCSP stapling.

https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_stapling
^ exists

https://nginx.org/en/docs/mail/ngx_mail_ssl_module.html#ssl_stapling
^ missing

When the configured certificate doesn't have OCSP information, it'll
just log a warning during startup.

core/nginx/conf/nginx.conf

@@ -58,6 +58,8 @@ http {
       listen [::]:443 ssl http2;
 
       include /etc/nginx/tls.conf;
+      ssl_stapling on;
+      ssl_stapling_verify on;
       ssl_session_cache shared:SSLHTTP:50m;
       add_header Strict-Transport-Security 'max-age=31536000';