Fix DefaultExcludesInProjectFolder value ignored (#24063)

* Fix DefaultExcludesInProjectFolder value ignored

The DefaultItemExcludesInProjectFolder property is never defined and was likely forgotten to be renamed. Because of that, the DefaultExcludesInProjectFolder property overwrites any user set value. Add DefaultExcludesInProjectFolder and keep DefaultItemExcludesInProjectFolder to not break existing consumers.

Author: ViktorHofer

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.DefaultItems.targets

@@ -36,9 +36,9 @@ Copyright (c) .NET Foundation. All rights reserved.
 
     <!-- WARNING: This pattern is there to ignore folders such as .git and .vs, but it will also match items included with a
          relative path outside the project folder (for example "..\Shared\Shared.cs").  So be sure only to apply it to items
-         that are in the project folder. -->
-    <DefaultExcludesInProjectFolder>$(DefaultItemExcludesInProjectFolder);**/.*/**</DefaultExcludesInProjectFolder>
-
+         that are in the project folder. Support both DefaultItemExcludesInProjectFolder and DefaultExcludesInProjectFolder
+         properties because of a naming mistake. -->
+    <DefaultExcludesInProjectFolder>$(DefaultExcludesInProjectFolder);$(DefaultItemExcludesInProjectFolder);**/.*/**</DefaultExcludesInProjectFolder>
   </PropertyGroup>
 
   <!-- Set the default versions of the NETStandard.Library or Microsoft.NETCore.App packages to reference.