xds/translator: migrate http-route-https-mtls-allow-expired-cert fixture to BackendClusterRefs schema

This fixture arrived via the sync with upstream main (PR #9199) after this
branch's earlier fixture-migration commits already ran, so it still used
the old inline destination.settings shape instead of
backendClusterRefs/backends. Migrate it the same way; golden output is
byte-for-byte identical, confirming this is a pure schema migration with
no behavior change.

Signed-off-by: Muhammad Waqar <waqar.hameed08@gmail.com>

Author: muwaqar

internal/xds/translator/testdata/in/xds-ir/http-route-https-mtls-allow-expired-cert.yaml

@@ -23,18 +23,22 @@ http:
   routes:
   - destination:
       name: httproute/default/httproute-1/rule/0
-      settings:
-      - addressType: IP
-        endpoints:
-        - host: 7.7.7.7
-          port: 8080
-        protocol: HTTP
-        weight: 1
-        name: httproute/default/httproute-1/rule/0/backend/0
+      backendClusterRefs:
+      - name: httproute/default/httproute-1/rule/0
     hostname: gateway.envoyproxy.io
     name: httproute/default/httproute-1/rule/0/match/0/gateway_envoyproxy_io
     pathMatch:
       distinct: false
       name: ""
       prefix: /
     useClientProtocol: true
+backends:
+- name: httproute/default/httproute-1/rule/0
+  settings:
+  - addressType: IP
+    endpoints:
+    - host: 7.7.7.7
+      port: 8080
+    protocol: HTTP
+    weight: 1
+    name: httproute/default/httproute-1/rule/0/backend/0