fix(traefik): replace invalid controllername flag with labelSelector for v3

Remove --providers.kubernetesgateway.controllername which does not exist in
Traefik v3, update GatewayClass controllerName to the standard v3 value, and
use labelSelector on each instance's kubernetesGateway provider to differentiate
internal vs external traffic.

apps/base/traefik-system/gatewayclass-external.yaml

@@ -4,4 +4,4 @@ kind: GatewayClass
 metadata:
   name: traefik-external
 spec:
-  controllerName: traefik.io/gateway-controller-external
+  controllerName: traefik.io/gateway-controller

apps/base/traefik-system/gatewayclass-internal.yaml

@@ -4,4 +4,4 @@ kind: GatewayClass
 metadata:
   name: traefik-internal
 spec:
-  controllerName: traefik.io/gateway-controller-internal
+  controllerName: traefik.io/gateway-controller

apps/overlays/au-syd1/traefik-system/values-external.yaml

@@ -1,8 +1,6 @@
 image:
   tag: v3.7.0
 
-additionalArguments:
-  - "--providers.kubernetesgateway.controllername=traefik.io/gateway-controller-external"
 
 podDisruptionBudget:
   enabled: true
@@ -29,6 +27,7 @@ providers:
     experimentalChannel: false
     namespaces: []
     nativeLBByDefault: false
+    labelSelector: "traefik.io/instance=external"
 
 logs:
   access:

apps/overlays/au-syd1/traefik-system/values-internal.yaml

@@ -1,8 +1,6 @@
 image:
   tag: v3.7.0
 
-additionalArguments:
-  - "--providers.kubernetesgateway.controllername=traefik.io/gateway-controller-internal"
 
 podDisruptionBudget:
   enabled: true
@@ -29,6 +27,7 @@ providers:
     experimentalChannel: false
     namespaces: []
     nativeLBByDefault: false
+    labelSelector: "traefik.io/instance=internal"
 
 logs:
   access: