Set backend_not_found route status when any backends are not found #4375

Job	Run time
meta	14s
info	1s
build-cli	2m 20s
build-ext (jaeger-webhook)	2m 9s
build-core (controller)	2m 29s
build-ext (metrics-api)	2m 5s
build-core (policy-controller)	5m 36s
build-core (proxy)	1m 57s
build-ext (tap)	2m 9s
build-ext (web)	3m 55s
test-policy (v1.22)	10m 19s
test-core (cni-calico-deep)	3m 34s
test-policy (v1.29)	9m 29s
test-core (deep)	3m 6s
build-ok	1s
test-ext (cluster-domain)	4m 6s
test-core (deep-native-sidecar)	4m 4s
test-ext (default-policy-deny)	3m 33s
test-ext (external)	5m 21s
test-viz	0s
test-multicluster	0s
test-ext (rsa-ca)	2m 55s
test-ext (helm-upgrade)	4m 47s
test-ext (uninstall)	3m 24s
test-ext (upgrade-edge)	4m 49s
integrations-ok	1s
integrations-retry	0s
	1h 22m 24s

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