refactor(persistence)!: group the values into existingPersistentVolumeClaim and new

The flat `persistence` dict mixed three concerns: whether persistence is used at all, whether the chart creates the
PersistentVolumeClaim, and how that claim is shaped. The pairs `create`/`claimName` and `enabled`/`mount` were only
meaningful in certain combinations, so an invalid configuration such as `create=true` together with a foreign
`claimName` was silently accepted. The same split into an `existingX`/`new` pair is already used for the Secrets, so
this aligns persistence with the rest of the chart.

`persistence.enabled` now only decides whether a volume is used at all. `persistence.existingPersistentVolumeClaim`
points at a claim managed outside of the chart, and everything under `persistence.new` describes the claim the chart
creates itself. Rendering and naming move into `templates/gitea/_persistentVolumeClaims.tpl` so the Deployment and the
PersistentVolumeClaim derive the claim name from a single helper instead of repeating the value lookups.

Support for `global.storageClass` is dropped. It was a chart-wide override that silently applied to the Gitea claim and
was evaluated through `tpl`, which made the effective storage class hard to predict. The storage class is now set
explicitly via `persistence.new.storageClassName`, which also matches the field name in the PersistentVolumeClaim spec.

BREAKING CHANGE: The `persistence` values were restructured and `global.storageClass` was removed.

- `persistence.create` and `persistence.mount` are gone. Set `persistence.enabled` to use a volume and
  `persistence.existingPersistentVolumeClaim.enabled` to reuse a claim that is not managed by the chart.
- `persistence.claimName` moves to `persistence.existingPersistentVolumeClaim.persistentVolumeClaimName`. A claim
  created by the chart is now named after `gitea.fullname` instead of the default `gitea-shared-storage`.
- `persistence.accessModes`, `annotations`, `labels`, `size` and `subPath` move into `persistence.new`.
- `persistence.volumeName` becomes `persistence.new.persistentVolumeName`.
- `persistence.storageClass` and `global.storageClass` become `persistence.new.storageClassName`.
- `persistence.enabled` now defaults to `false`.

Co-authored-by: Copilot <copilot@github.com>

templates/gitea/secret_config.yaml

@@ -37,8 +37,8 @@ stringData:
       {{- end }}
     {{- end }}
 
-    {{- if eq (first .Values.persistence.accessModes) "ReadWriteOnce" -}}
-      {{- fail "When using multiple replicas, a RWX file system is required and persistence.accessModes[0] must be set to ReadWriteMany." -}}
+    {{- if eq (first .Values.persistence.new.accessModes) "ReadWriteOnce" -}}
+      {{- fail "When using multiple replicas, a RWX file system is required and persistence.new.accessModes[0] must be set to ReadWriteMany." -}}
     {{- end }}
     {{- if .Values.gitea.config.indexer -}}
       {{- if eq .Values.gitea.config.indexer.ISSUE_INDEXER_TYPE "bleve" -}}