service: use selectorLabels helper

[craigcabrey]

fixes #93

before:

$ helm template release ./vaultwarden
[snip]
apiVersion: v1
kind: Service
metadata:
  name: release-vaultwarden
  namespace: default
  labels:
    app.kubernetes.io/component: vaultwarden
    helm.sh/chart: vaultwarden-0.23.0
    app.kubernetes.io/name: vaultwarden
    app.kubernetes.io/instance: release
    app.kubernetes.io/version: "1.30.3"
    app.kubernetes.io/managed-by: Helm
spec:
  type: "ClusterIP"
  selector:
    app.kubernetes.io/component: vaultwarden
  ports:
    - name: "http"
      port: 80
      protocol: TCP
      targetPort: 8080
    - name: "websocket"
      port: 3012
      protocol: TCP
      targetPort: 3012
  ipFamilyPolicy: SingleStack
[snip]

after:

$ helm template release ./vaultwarden
[snip]
kind: Service
metadata:
  name: release-vaultwarden
  namespace: default
  labels:
    app.kubernetes.io/component: vaultwarden
    helm.sh/chart: vaultwarden-0.23.0
    app.kubernetes.io/name: vaultwarden
    app.kubernetes.io/instance: release
    app.kubernetes.io/version: "1.30.3"
    app.kubernetes.io/managed-by: Helm
spec:
  type: "ClusterIP"
  selector:
    app.kubernetes.io/name: vaultwarden
    app.kubernetes.io/instance: release
[snip]