arrproxy: use real quay.io oauth2-proxy address (rke2 mirror)

Repoints the oauth2-proxy sidecar image from the one-off artifactapi
copy to the real upstream address quay.io/oauth2-proxy/oauth2-proxy,
relying on the rke2 registries.yaml mirror that rewrites quay.io to
artifactapi.k8s.syd1.au.unkin.net/quay.

apps/base/arrstack/arrproxy/oauth2-proxy-deployment.yaml

@@ -62,7 +62,7 @@ spec:
               memory: 64Mi
       containers:
         - name: oauth2-proxy
-          image: artifactapi.k8s.syd1.au.unkin.net/ghcr/oauth2-proxy/oauth2-proxy:v7.15.3
+          image: quay.io/oauth2-proxy/oauth2-proxy:v7.15.3
           imagePullPolicy: IfNotPresent
           ports:
             - containerPort: 4180