upgrade .net 5

MergeSort_Array/MergeSort_Array/MergeSort_Array.csproj

 <Project Sdk="Microsoft.NET.Sdk">
-
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
   </PropertyGroup>
-
-</Project>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.DotNet.UpgradeAssistant.Extensions.Default.Analyzers" Version="0.3.246501">
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+</Project>
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