feat: Add support for setting priorityClassName (#430)

### Description of the change

Adds support for setting priorityClassName on the Gitea pod.

### Benefits

Users can take advantage of https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/ with Gitea.

### Applicable issues

  - fixes #429

### Checklist

<!-- [Place an '[X]' (no spaces) in all applicable fields. Please remove unrelated fields.] -->

- [x] Parameters are documented in the `values.yaml` and added to the `README.md` using [readme-generator-for-helm](https://github.com/bitnami-labs/readme-generator-for-helm)

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/430
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Co-authored-by: Casey Buto <cbuto@d2iq.com>
Co-committed-by: Casey Buto <cbuto@d2iq.com>

templates/gitea/statefulset.yaml

@@ -39,6 +39,9 @@ spec:
       {{- if .Values.schedulerName }}
       schedulerName: "{{ .Values.schedulerName }}"
       {{- end }}
+      {{- if .Values.priorityClassName }}
+      priorityClassName: "{{ .Values.priorityClassName }}"
+      {{- end }}
       {{- include "gitea.images.pullSecrets" . | nindent 6 }}
       securityContext:
         {{- toYaml .Values.podSecurityContext | nindent 8 }}