Fix Deeply Nesting UnmodifableSet Wrappers in UnassignedInfo

The linked issue is pretty fringe, but even during normal operation we keep nesting these wrappers a bunch of

closes elastic#76490

server/src/main/java/org/elasticsearch/cluster/routing/UnassignedInfo.java

@@ -241,7 +241,7 @@ public UnassignedInfo(Reason reason, @Nullable String message, @Nullable Excepti
         this.failure = failure;
         this.failedAllocations = failedAllocations;
         this.lastAllocationStatus = Objects.requireNonNull(lastAllocationStatus);
-        this.failedNodeIds = Collections.unmodifiableSet(failedNodeIds);
+        this.failedNodeIds = Set.copyOf(failedNodeIds);
         assert (failedAllocations > 0) == (reason == Reason.ALLOCATION_FAILED) :
             "failedAllocations: " + failedAllocations + " for reason " + reason;
         assert (message == null && failure != null) == false : "provide a message if a failure exception is provided";