Add wekan kustomisation

flux2/cluster/optional/wekan/wekan.yaml

+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
+kind: Kustomization
+metadata:
+  name: wekan
+  namespace: flux-system
+spec:
+  interval: 1h
+  dependsOn:
+    - name: core
+    - name: infrastructure
+  sourceRef:
+    kind: GitRepository
+    name: openappstack
+  path: ./flux2/apps/wekan
+  prune: true
+  validation: client
+  postBuild:
+    substituteFrom:
+      - kind: Secret
+        name: oas-wekan-variables
+      - kind: Secret
+        name: oas-oauth-variables
+      - kind: Secret
+        name: oas-cluster-variables