Bitmap class should have transparency informations when loading from file

* .NET Core Version:

.NET SDK (gemäß "global.json"):
Version: 5.0.103
Commit: 72dec52dbd

Laufzeitumgebung:
OS Name: Windows
OS Version: 10.0.19042
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.103\

Host (useful for support):
Version: 5.0.3
Commit: c636bbdc8a

* Have you experienced this same bug with .NET Framework?:
Yes

**Problem description:**
Bitmap class has no transparency informations (alpha channel) when loading from file or stream with an alpha channel..

**Expected behavior:**
The left side is the current situation. The right side is the expected one.

**Minimal repro:**
[TestBitmap.zip](https://github.com/dotnet/winforms/files/5960339/TestBitmap.zip)
See possible solution in file AlphaBitmap.cs