Requests being routed to nonexistent backend whitehall-frontend.

[sengi]

We've been logging about 1100 of these per day since whitehall-frontend was decommissioned:

Representative stack trace:

*net.OpError: dial tcp: lookup whitehall-frontend.production.govuk-internal.digital on 172.20.0.10:53: no such host
  File "github.com/alphagov/router/logger/sentry.go", line 74, in NotifySentry
  File "github.com/alphagov/router/handlers/backend_handler.go", line 164, in (*backendTransport).RoundTrip
  File "net/http/httputil/reverseproxy.go", line 473, in (*ReverseProxy).ServeHTTP
  File "github.com/alphagov/router/triemux/mux.go", line 50, in (*Mux).ServeHTTP
  File "github.com/alphagov/router/lib/router.go", line 150, in (*Router).ServeHTTP
  File "net/http/server.go", line 2938, in serverHandler.ServeHTTP
  File "net/http/server.go", line 2009, in (*conn).serve

Sentry issue for cross-referencing (internal-only, sorry)

Suspect there's some dangling bit of config somewhere, at least a dead route in the Router database but there may well be other cleanup upstream that was missed.

[sengi]

db.routes.find({handler: 'backend', backend_id: 'whitehall-frontend'});
{
  "_id" : ObjectId("5bb7754145a5a0728ceadbb7"),
  "incoming_path" : "/government/topics/transport",
  "disabled" : false,
  "route_type" : "exact",
  "handler" : "backend",
  "redirect_to" : "/transport",
  "redirect_type" : "permanent",
  "segments_mode" : "ignore",
  "updated_at" : ISODate("2023-09-20T06:45:25.047Z"),
  "created_at" : ISODate("2018-10-05T14:29:21.791Z"),
  "backend_id" : "whitehall-frontend"
}
...

db.routes.find({handler: 'backend', backend_id: 'whitehall-frontend'}).count();
30

These all look like they're supposed to be redirects (because of the redirect_to field), but they have route_type: backend instead of route_type: redirect.