Document that Tunnels are PQ by default, and how they can be configured in strict mode

[lukevalenta]

Existing documentation URL(s)

• https://developers.cloudflare.com/cloudflare-one/connections/connect-networks/

What changes are you suggesting?

Document that the current behavior of Cloudflare Tunnel is to prefer post-quantum by default, but to fall back to non-PQ if there are issues connecting: https://github.com/cloudflare/cloudflared/blob/11777db3047bccda1ca7e2b27c3ceeab4ba3fa97/features/features.go#L31. If the --post-quantum flag is provided, there is no fallback to non-PQ.

Additional information

History:

• Version 2022.9.1 added PQ support when the --post-quantum flag is provided (https://blog.cloudflare.com/post-quantum-tunnel/).
• Version 2024.1.0 uses post-quantum by default (for QUIC connections, at least), changing the meaning of the --post-quantum flag to be "Strict" post-quantum with no fallback to non-PQ. (commit: cloudflare/cloudflared@4ddc8d7)

[ranbel]

tracked internally in PCX-16400