default ALPN settings break TLS-terminated TCP route

[guydc]

Description:

Envoy Gateway sets ALPN to h2, http/1.1 by default. When Envoy is not handling the application layer protocol (e.g. TLS termination for TCP), Envoy should not advertise application layer protocols that may not be supported by the backend, and keep the downstream TLS socket ALPN settings undefined.

In general, it seems that Envoy Gateway should perhaps support more flexibility around ALPN:

• Supporting non-HTTP ALPN protos for TLS routes that proxy non-HTTP protos
• Supporting ALPN disablement
• Changing the defaults for non-HTTP routes

Repro steps:
Follow the the official guide: https://gateway.envoyproxy.io/docs/tasks/security/tls-termination/

curl -v -HHost:www.example.com --resolve "www.example.com:8443:127.0.0.1"  --cacert example.com.crt https://www.example.com:8443/get
* Added www.example.com:8443:127.0.0.1 to DNS cache
* Hostname www.example.com was found in DNS cache
*   Trying 127.0.0.1:8443...
* Connected to www.example.com (127.0.0.1) port 8443
* ALPN: server accepted h2
[...]
* using HTTP/2
* [HTTP/2] [1] OPENED stream for https://www.example.com:8443/get
* [HTTP/2] [1] [:method: GET]
* [HTTP/2] [1] [:scheme: https]
* [HTTP/2] [1] [:authority: www.example.com]
* [HTTP/2] [1] [:path: /get]
* [HTTP/2] [1] [user-agent: curl/8.7.1]
* [HTTP/2] [1] [accept: */*]
> GET /get HTTP/2
> Host:www.example.com
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
* Remote peer returned unexpected data while we expected SETTINGS frame.  Perhaps, peer does not support HTTP/2 properly.

Envoy encourages the client the use HTTP/2, which is not supported by the upstream server, leading to a failure. If the client is explicitly using HTTP/1, the request succeeds:

curl -v --http1.1 -HHost:www.example.com --resolve "www.example.com:8443:127.0.0.1" --cacert example.com.crt https://www.example.com:8443/get
[...]
* using HTTP/1.x
> GET /get HTTP/1.1
> Host:www.example.com
> User-Agent: curl/8.7.1
> Accept: */*
> 
* Request completely sent off
< HTTP/1.1 200 OK