staging: Adapt loader directory usage

[1] https://grafana.softwareheritage.org/goto/nLpc3rUVz?orgId=1

Refs. swh/infra/sysadm-environment#4883

swh/values/staging.yaml

@@ -60,7 +60,7 @@ loaders:
         maxReplicaCount: 1
       image: swh_loader_package_image
     directory:
-      requestedMemory: 256Mi
+      requestedMemory: 768Mi
       requestedCpu: 200m
       queues:
         - swh.loader.core.tasks.LoadDirectory