refactor(serviceAccount)!: group the values into existingServiceAccount and new

The previous notation mixed two concerns in a single flat dict: `create`/`name` decided whether the chart manages the
ServiceAccount or only references an externally provided one, while the remaining keys only ever applied to a
chart-managed ServiceAccount. That made `name` ambiguous, because it either renamed the generated object or pointed to a
foreign one, and it forced the deployment to guard the reference with `or .Values.serviceAccount.create
.Values.serviceAccount.name`.

The values are now grouped the same way as `persistence`, which was restructured in dfe087c. `serviceAccount.enabled`
controls whether the pod uses a ServiceAccount at all, `serviceAccount.existingServiceAccount` references an externally
managed object and `serviceAccount.new` holds the properties of the object created by the chart. The templates follow
the established helper layout in `templates/gitea/_serviceAccounts.tpl`.

Two latent bugs are fixed along the way. The annotation helper was defined as `gitea.secret.admin.annotations`, so the
`gitea.serviceAccount.annotations` include in the template never resolved. And the duplicated name helper
`gitea.serviceAccountName` in `templates/_helpers.tpl` is removed in favour of `gitea.serviceAccount.name`.

BREAKING CHANGE:
- `serviceAccount.create` does no longer exist. Use `serviceAccount.enabled` instead. The default changed from `false`
  to `true`, so a ServiceAccount is now created unless it is explicitly disabled.
- `serviceAccount.name` does no longer exist. Use `serviceAccount.existingServiceAccount.enabled` together with
  `serviceAccount.existingServiceAccount.existingServiceAccountName` to reference an externally managed ServiceAccount.
- `serviceAccount.annotations`, `serviceAccount.labels`, `serviceAccount.automountServiceAccountToken` and
  `serviceAccount.imagePullSecrets` moved below `serviceAccount.new`.

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

templates/gitea/deployment.yaml

@@ -42,8 +42,8 @@ spec:
       {{- if .Values.deployment.schedulerName }}
       schedulerName: "{{ .Values.deployment.schedulerName }}"
       {{- end }}
-      {{- if (or .Values.serviceAccount.create .Values.serviceAccount.name) }}
-      serviceAccountName: {{ include "gitea.serviceAccountName" . }}
+      {{- if .Values.serviceAccount.enabled }}
+      serviceAccountName: {{ include "gitea.serviceAccount.name" . }}
       {{- end }}
       {{- if .Values.deployment.priorityClassName }}
       priorityClassName: "{{ .Values.deployment.priorityClassName }}"