Set Strict-Transport-Security header in .htaccess

[schmidt1024]

Enforce HTTPS by configuring Strict-Transport-Security header

This change addresses the recommendation in the Nextcloud Documentation and admin panel to enable HTTP Strict Transport Security.

For more information, refer to the official Nextcloud documentation: https://docs.nextcloud.com/server/latest/admin_manual/installation/harden_server.html#enable-http-strict-transport-security