Emits PostgREST v16.0's deprecation Warning header for legacy embed target names.

When a select aliases an embedded resource (the_tasks:tasks(...)) a filter, order or limit may still address it by the relation name (tasks.name=like.Code*). v16.0 kept that working under the default url-use-legacy-target-names = true but marked it deprecated: the read plan records the legacy match (relIsLegacyTargetNameMatch, Plan.hs), and App.hs' toWaiResponse appends one header naming every replacement:

299 <product-token> "Embedded resource was referenced by relation name
even though it has an alias. This is deprecated and will stop working in a
future release. Update `tasks` to `the_tasks` in query string filters,
orders or limits."

Two properties are reproduced here:

• One header per response, from a single funnel. Upstream builds it in toWaiResponse from the whole plan, so several legacy matches collapse into one header whose replacement list is comma-joined — hence a register_before_send/2 callback fed by record/2 rather than a header set at each rendering site.

• It is a deprecation notice, not an error. Addressing the embed by its alias sets no flag and emits no header, and with url-use-legacy-target-names = false the request is rejected outright instead of warned about (so nothing is recorded either).

299 is the RFC 7234 "miscellaneous persistent warning" code; the product token identifies the emitting server (upstream uses PostgRESTv<version>).