Upgrade .NET Core 3.1 to .NET 5 (#15591)

* Common.UI

* ColorPicker

* PT Run

* File Explorer Add-ons

* Awake

* FZ Editor

* ImageResizer

* Interop

* Docs

* Installer

* Fix test not being run - Downgrade MSTest.TestAdapter & MSTest.TestFramework

* Update expect.txt

* Test run fix

src/modules/previewpane/PdfPreviewHandler/PdfPreviewHandler.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Platforms>x64</Platforms>
     <UseWindowsForms>true</UseWindowsForms>
@@ -23,7 +23,7 @@
   <PropertyGroup>
     <ProjectGuid>{69E1EE8D-143A-4060-9129-4658ACF14AAF}</ProjectGuid>
     <RootNamespace>Microsoft.PowerToys.PreviewHandler.Pdf</RootNamespace>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0-windows10.0.18362.0</TargetFramework>
     <EnableComHosting>true</EnableComHosting>
     <IntermediateOutputPath>$(SolutionDir)$(Platform)\$(Configuration)\obj\$(AssemblyName)\</IntermediateOutputPath>
     <AssemblyName>PowerToys.PdfPreviewHandler</AssemblyName>
@@ -49,7 +49,6 @@
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
     <PackageReference Include="System.IO.Abstractions" Version="12.2.5" />
-    <PackageReference Include="System.Runtime.WindowsRuntime" Version="4.7.0" />
   </ItemGroup>
   
   <ItemGroup>
@@ -62,11 +61,4 @@
     <ProjectReference Include="..\..\..\common\ManagedTelemetry\Telemetry\ManagedTelemetry.csproj" />
     <ProjectReference Include="..\common\PreviewHandlerCommon.csproj" />
   </ItemGroup>
-   
-  <ItemGroup>
-    <Reference Include="Windows">
-      <HintPath>$(MSBuildProgramFiles32)\Windows Kits\10\UnionMetadata\10.0.18362.0\Windows.winmd</HintPath>
-      <IsWinMDFile>true</IsWinMDFile>
-    </Reference>
-  </ItemGroup>
 </Project>

src/modules/previewpane/PdfPreviewHandler/PdfPreviewHandlerControl.cs

@@ -207,7 +207,7 @@ namespace Microsoft.PowerToys.PreviewHandler.Pdf
         /// <returns>An object of type <see cref="Image"/></returns>
         private Image PageToImage(PdfPage page)
         {
-            Image imageOfPage;
+            Image imageOfPage = null;
 
             using (var stream = new InMemoryRandomAccessStream())
             {