[IMP] doc: modernize NGINX SSL recommandations

TLS 1 and 1.1 are not recommended anymore
This configuration was generated with Mozilla SSL Configuration
Generator (https://ssl-config.mozilla.org

) with a fairly conservative
approach:
- nginx 1.10.3, OpenSSL 1.1.0l (Debian Stretch)
- Supports Firefox 27, Android 4.4.2, Chrome 31, Edge, IE 11 on
  Windows 7, Java 8u31, OpenSSL 1.0.1, Opera 20, and Safari 9

Fixes odoo/odoo#68002

closes odoo/odoo#68207

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>