Fix peerauthentication having incorrect selectorLabels

Kiali is throwing an error: peerauth is using a bad match label selector within the file: https://repo1.dso.mil/big-bang/product/packages/external-secrets/-/blob/main/chart/templates/bigbang/istio/peerauthentication/external-secrets.yaml

Solution

Replace

app: external-secrets 

with

{{- include "external-secrets.selectorLabels" . | nindent 6 }}