feat(matrix): Add additional database replica

This patch adds an additional replica of the database, which should help
to prevent reduce risks of a single Database being down.

apps/k8s01/matrix/database-override.yaml

+apiVersion: "acid.zalan.do/v1"
+kind: postgresql
+metadata:
+  name: matrix-postgres
+  namespace: matrix
+spec:
+  numberOfInstances: 2

apps/k8s01/matrix/kustomization.yaml

@@ -7,4 +7,5 @@ resources:
   - signing-key.yaml
   - ../../../shared/resourcequotas/default.yaml
 patchesStrategicMerge:
+  - database-override.yaml
   - matrix-synapse-values.yaml