Use System.Threading.Lock

Use `System.Threading.Lock` instead of an object to resolve `IDE0330` warning in .NET 9 preview 7.

src/ProjectEuler/Puzzles/Puzzle014.cs

@@ -13,7 +13,7 @@ public sealed class Puzzle014 : Puzzle
     /// </summary>
     private static readonly Dictionary<long, int> _cache = new(1_000_000);
 
-    private static readonly object _lock = new();
+    private static readonly System.Threading.Lock _lock = new();
 
     /// <inheritdoc />
     public override string Question => "Which starting number, under one million, produces the longest chain?";