Fix build

Avoid using ValueRef with older framework versions.

Author: quzary

bench/algorithm/lru/2.cs

@@ -57,7 +57,11 @@ public bool TryGet(TK key, out TV value)
     {
         if (_key_lookup.TryGetValue(key, out var node))
         {
+#if NET5_0_OR_GREATER
             value = node.ValueRef.Value;
+#else
+            value = node.Value.Value;
+#endif
             _entries.Remove(node);
             _entries.AddLast(node);
             return true;