Marked Singleton<T> obsolete

This pattern is discouraged.

... I regret adding it.

CHANGELOG.md

@@ -44,6 +44,9 @@
 - X10D.Unity: Added `Vector3Int.WithZ()`
 - X10D.Unity: Added `Vector4.Deconstruct()`
 
+### Changed
+- X10D.Unity: Obsolesced `Singleton<T>`
+
 ## [3.1.0]
 ### Added
 - Reintroduced Unity support

X10D.Unity/src/Singleton.cs

@@ -7,6 +7,8 @@ namespace X10D.Unity;
 ///     thread-safe.
 /// </summary>
 /// <typeparam name="T">The type of the singleton.</typeparam>
+[Obsolete("This implementation of the singleton pattern is discouraged, and this class will be removed in future. " +
+          "DO NOT USE THIS TYPE IN PRODUCTION.")]
 public abstract class Singleton<T> : MonoBehaviour
     where T : Singleton<T>
 {