consider changing the implementation of the Envoy readiness probe

Currently, the Envoy readiness probe hits the `/ready` endpoint on the Envoy admin interface. (https://github.com/projectcontour/contour/blob/main/examples/contour/03-envoy.yaml#L82-L87)

https://github.com/envoyproxy/envoy/issues/16425 documents potential issues with using this endpoint for health-checking Envoy, due to its handler running on the main thread.

We should consider changing the implementation of the readiness probe. One option would be to set up a static HTTP listener that serves a direct response specifically for health-checking.

Start with examples/contour/03-envoy.yaml and the current `/ready` probe, then read Envoy issue 16425 to understand the health-checking concern. Done means selecting and validating an alternative readiness-probe implementation, with the affected deployment configuration and tests or validation steps identified.

Written by the indexing model from the issue text.