Replaced Span<T>.Fill(0) with Span<T>.Clear()

Cf. dotnet/aspnetcore#20023 (comment)

Author: DotDeveloper95

src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Internal/Sequence`1.cs

@@ -447,7 +447,7 @@ private void ClearReferences(int startIndex, int length)
                 // If we store references, clear them to allow the objects to be GC'd.
                 if (!IsValueTypeElement)
                 {
-                    this.AvailableMemory.Span.Slice(startIndex, length).Fill(default);
+                    this.AvailableMemory.Span.Slice(startIndex, length).Clear();
                 }
             }
         }