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

.pipelines/ci/templates/build-powertoys-steps.yml

@@ -124,6 +124,7 @@ steps:
       **\Microsoft.PowerToys.Run.Plugin.System.UnitTests.dll     
       **\Microsoft.PowerToys.Run.Plugin.WindowsTerminal.UnitTests.dll
       !**\obj\**
+      !**\ref\**
 
 # Native dlls
 - task: VSTest@2