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>

README.md

@@ -672,6 +672,7 @@ gitea:
 | `tolerations`                               | Tolerations for the statefulset                        | `[]`  |
 | `affinity`                                  | Affinity for the statefulset                           | `{}`  |
 | `dnsConfig`                                 | dnsConfig for the statefulset                          | `{}`  |
+| `priorityClassName`                         | priorityClassName for the statefulset                  | `""`  |
 | `statefulset.env`                           | Additional environment variables to pass to containers | `[]`  |
 | `statefulset.terminationGracePeriodSeconds` | How long to wait until forcefully kill the pod         | `60`  |
 | `statefulset.labels`                        | Labels for the statefulset                             | `{}`  |