NGINX settings: clarify the use of HSTS

Warning

The NGINX configuration will tell browsers and clients to only communicate with your GitLab instance over a secure connection for the next 24 months. By enabling HTTPS you’ll need to provide a secure connection to your instance for at least the next 24 months.

• https://docs.gitlab.com/omnibus/settings/nginx.html#warning

This is achieved by providing the following HTTP header (a.k.a HSTS):

strict-transport-security: max-age=63072000