fix: only use one secret for weave-gitops and make README more clear on usage

Signed-off-by: Devin Buhl <devin@buhl.casa>

tmpl/kubernetes/weave-gitops-secret.sops.yaml

@@ -1,8 +1,10 @@
+---
 apiVersion: v1
 kind: Secret
 metadata:
-  name: weave-gitops-secret
+  name: cluster-user-auth
   namespace: flux-system
 type: Opaque
 stringData:
-  adminPassword: "${BOOTSTRAP_WEAVE_GITOPS_ADMIN_PASSWORD}"
+  username: admin
+  password: "${BOOTSTRAP_WEAVE_GITOPS_ADMIN_PASSWORD}"