fix: frontend PodDisruptionBudget (#633)

### Checklist

- [x] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [x] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [x] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

Unit test.
e2e on cluster.

charts/sourcegraph/CHANGELOG.md

@@ -8,6 +8,8 @@ Use `**BREAKING**:` to denote a breaking change
 
 ## Unreleased
 
+- Fix Pod Disruption Budget for sourcegraph-frontend
+
 ## 5.10.0
 
 - Updated OpenTelemetry collector and agent images to run as non-root users [#543](https://github.com/sourcegraph/deploy-sourcegraph-helm/pull/543)

charts/sourcegraph/templates/frontend/sourcegraph-frontend.PodDisruptionBudget.yaml

@@ -6,6 +6,7 @@ metadata:
 spec:
   {{- toYaml .Values.frontend.podDisruptionBudget | nindent 2 }}
   selector:
-    {{- include "sourcegraph.selectorLabels" . | nindent 4 }}
-    app: sourcegraph-frontend
+    matchLabels:
+      {{- include "sourcegraph.selectorLabels" . | nindent 6 }}
+      app: sourcegraph-frontend
 {{- end }}

charts/sourcegraph/tests/podDisruptionBudget_test.yaml

@@ -13,3 +13,6 @@ tests:
   - equal:
       path: spec.minAvailable
       value: 1
+  - equal:
+      path: spec.selector.matchLabels.app
+      value: sourcegraph-frontend