Update architecture.md of demo to reflect actual protocols used

[ra2xfael]

This PR updates the service graph for the documentation of the OpenTelemetry Demo Application to show that the communication between frontend → shipping and checkout → shipping actually uses gRPC instead of HTTP. I verified this using curl, so this change should make the demo documentation match what’s really happening.

I reported this deviation under #8165

Here is my test:

$ curl --http2-prior-knowledge -v http://shipping:8080/
* Host shipping:8080 was resolved.
* IPv6: (none)
* IPv4: 10.109.228.246
*   Trying 10.109.228.246:8080...
* Established connection to shipping (10.109.228.246 port 8080) from 10.244.3.109 port 43922 
* using HTTP/2
* [HTTP/2] [1] OPENED stream for http://shipping:8080/
* [HTTP/2] [1] [:method: GET]
* [HTTP/2] [1] [:scheme: http]
* [HTTP/2] [1] [:authority: shipping:8080]
* [HTTP/2] [1] [:path: /]
* [HTTP/2] [1] [user-agent: curl/8.16.0]
* [HTTP/2] [1] [accept: */*]
> GET / HTTP/2
> Host: shipping:8080
> User-Agent: curl/8.16.0
> Accept: */*
> 
* Request completely sent off
< HTTP/2 200 
< grpc-status: 12
< content-type: application/grpc
< content-length: 0
< date: Sun, 19 Oct 2025 14:45:30 GMT
< 
* Connection #0 to host shipping:8080 left intact

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: ra2xfael / name: Raffael Huwe (68cfb6f)

[svrnm]

@open-telemetry/demo-approvers PTAL

[julianocosta89]

Hello @ra2xfael, thanks for raising this PR, but the current protocol is actually correct.

The Shipping service was rewritten using the actix-web Rust framework, and it is using HTTP, not gRPC.

Shipping service is only using the OTLP gRPC exporter, but the communication between frontend and checkout services with shipping is all done via HTTP.

[julianocosta89]

@ra2xfael maybe you are using an old version of the Demo.
The rewrite happened on this PR: open-telemetry/opentelemetry-demo#2214

[ra2xfael]

Indeed, I used version 2.0.2 of the demo which does not include PR #2214