Motivation for this change

This commit adds a Strict-Transport-Security header to
the nginx config file generated by the nextcloud module.

The Strict-Transport-Security header is recommended in
official guide for hardening Nextcloud installations:
https://docs.nextcloud.com/server/16/admin_manual/installation/harden_server.html

Further, if it is not set, we see a warning in the security scan results
in the Nextcloud admin panel:

The "Strict-Transport-Security" HTTP header is not set to at least "15552000" seconds. For enhanced security, it is recommended to enable HSTS as described in the security tips

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

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