fix: correct data type for restic secret

restic/templates/secret.yaml

 apiVersion: v1
-kind: ConfigMap
+kind: Secret
 metadata:
   name: {{ include "restic-helm.fullname" . }}
   labels:
     {{- include "restic-helm.labels" . | nindent 4 }}
-data:
+stringData:
   RESTIC_PASSWORD: "{{ .Values.repository.key }}"
   RESTIC_REPOSITORY: "{{ .Values.repository.name }}"
   {{- .Values.auth | nindent 2 }}