remove deprecated start_child_span option

[puckpuck]

Changes

Removes a deprecated option from the Envoy config. This removes the following warning from the frontend-proxy logs on startup.

frontend-proxy  | [2024-03-19 18:00:53.063][8][info][tracing] [source/common/tracing/tracer_manager_impl.cc:42] instantiating a new tracer: envoy.tracers.opentelemetry
frontend-proxy  | [2024-03-19 18:00:53.063][8][warning][misc] [source/common/protobuf/message_validator_impl.cc:21] Deprecated field: type envoy.extensions.filters.http.router.v3.Router Using deprecated option 'envoy.extensions.filters.http.router.v3.Router.start_child_span' from file router.proto. This configuration will be removed from Envoy soon. Please see https://www.envoyproxy.io/docs/envoy/latest/version_history/version_history for details. If continued use of this field is absolutely necessary, see https://www.envoyproxy.io/docs/envoy/latest/configuration/operations/runtime#using-runtime-overrides-for-deprecated-features for how to apply a temporary and highly discouraged override.
frontend-proxy  | [2024-03-19 18:00:53.063][8][warning][misc] [source/common/protobuf/message_validator_impl.cc:21] Deprecated field: type envoy.extensions.filters.http.router.v3.Router Using deprecated option 'envoy.extensions.filters.http.router.v3.Router.start_child_span' from file router.proto. This configuration will be removed from Envoy soon. Please see https://www.envoyproxy.io/docs/envoy/latest/version_history/version_history for details. If continued use of this field is absolutely necessary, see https://www.envoyproxy.io/docs/envoy/latest/configuration/operations/runtime#using-runtime-overrides-for-deprecated-features for how to apply a temporary and highly discouraged override.

We already use the spawn_upstream_span option configured within the tracing portion of the envoy filter, as recommended by Envoy docs, so tracing still works as expected.

Merge Requirements

For new features contributions please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.