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>

unittests/helm/persistentVolumeClaim/persistentVolumeClaim.yaml

@@ -0,0 +1,29 @@
+chart:
+  appVersion: 1.27.3 # renovate: datasource=docker registryUrl=https://docker.gitea.com depName=gitea
+release:
+  name: gitea-unittests
+  namespace: testing
+suite: PVC template
+templates:
+  - templates/gitea/persistentVolumeClaim.yaml
+tests:
+  - it: Skip persistentVolumeClaim if persistentVolumeClaim is disabled
+    set:
+      persistence.enabled: false
+    asserts:
+      - hasDocuments:
+          count: 0
+
+  - it: Render persistentVolumeClaim with default values
+    set:
+      persistence.enabled: true
+      persistence.new.storageClassName: "my-storage-class"
+    asserts:
+      - containsDocument:
+          apiVersion: v1
+          kind: PersistentVolumeClaim
+          name: gitea-unittests
+          namespace: testing
+      - equal:
+          path: spec.storageClassName
+          value: "my-storage-class"