ReferenceEqualityComparer is ambigous with .NET 5.0

There would be a problem once library is compiled with .NET 5.0.

It should be under `#if less than NET5.0`

https://docs.microsoft.com/en-us/dotnet/api/system.collections.generic.referenceequalitycomparer?view=net-5.0

在儲存庫中搜尋 ReferenceEqualityComparer，並檢查其針對目標 Framework 的前置處理器條件。驗證針對 .NET 5.0 編譯時的歧義，然後確認針對該目標會排除 comparer，且專案仍能針對受支援的 Framework 建置。

由索引模型根據 Issue 內容生成。