The cache demo returns 503 when the cached response is no longer valid

*Title*: The [cache demo](https://www.envoyproxy.io/docs/envoy/v1.27.0/start/sandboxes/cache) returns 503 when the cached response is no longer valid

*Description*:
The demo works fine as long as the response is either not cached or the cached response is still valid. Once the cached answer times out or requires validation, Envoy replies with 503.

*Repro steps*:
Follow the instructions from the [documentation](https://www.envoyproxy.io/docs/envoy/v1.27.0/start/sandboxes/cache):
```
$ cd examples/cache
$ docker compose pull
$ docker compose up --build -d
```
The no-cache example is a good demonstrator of this behaviour:
```
$ curl -i localhost:8000/service/1/no-cache
HTTP/1.1 200 OK
content-type: text/plain; charset=utf-8
cache-control: max-age=0, no-cache
etag: "1621f3c428db2d63d351830b17508da8bfabfca0"
content-length: 125
date: Thu, 17 Aug 2023 14:13:24 GMT
server: envoy
x-envoy-upstream-service-time: 1

This response can be cached, but it has to be validated on each request
Response generated at: Thu, 17 Aug 2023 14:13:24 GMT

$ curl -i localhost:8000/service/1/no-cache
HTTP/1.1 503 Service Unavailable
content-length: 95
content-type: text/plain
date: Thu, 17 Aug 2023 14:13:24 GMT
server: envoy

upstream connect error or disconnect/reset before headers. reset reason: connection termination%
```