Add app and version labels (#121)

This pull request adds the `app` and `version` labels that are used by Istio.

> Pods with app and version labels: We recommend adding an explicit app label and version label to the specification of the pods deployed using a Kubernetes Deployment. The app and version labels add contextual information to the metrics and telemetry that Istio collects.
>
> * The app label: Each deployment should have a distinct app label with a meaningful value. The app label is used to add contextual information in distributed tracing.
>
> * The version label: This label indicates the version of the application corresponding to the particular deployment.

From https://istio.io/latest/docs/ops/deployment/requirements/#pod-requirements

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/121
Reviewed-by: luhahn <luhahn@noreply.gitea.io>
Reviewed-by: lafriks <lafriks@noreply.gitea.io>
Co-authored-by: Starefossen <starefossen@noreply.gitea.io>
Co-committed-by: Starefossen <starefossen@noreply.gitea.io>

templates/gitea/statefulset.yaml

@@ -19,7 +19,7 @@ spec:
         {{- toYaml . | nindent 8 }}
         {{- end }}
       labels:
-        {{- include "gitea.selectorLabels" . | nindent 8 }}
+        {{- include "gitea.labels" . | nindent 8 }}
     spec:
       {{- with .Values.imagePullSecrets }}
       imagePullSecrets: