Update to .NET 5 (#10346)

* Update to .NET 5

.NET 5 brings many performance (especially regex) improvements
https://devblogs.microsoft.com/dotnet/announcing-net-5-0/

Can look at bringing the packages upto date and using a single file executable in a future PR if the maintainers are interested

* Dotnet restore before building

* Restore on Windows only

* Out of ideas

* Update framework for windows specific apps

* Don't upgrade Microsoft.Extensions.Configuration

* drop mono from the dotnet linux build instructions

* Fix logic to get app runtime

* readme: add windows .net prereq link

* Update README.md

* azure-pipeline: bump minorversion to 17

Co-authored-by: garfield69 <garfieldsixtynine@gmail.com>
Co-authored-by: Diego Heras <ngosang@hotmail.es>
Co-authored-by: ilike2burnthing <59480337+ilike2burnthing@users.noreply.github.com>

src/Jackett.IntegrationTests/Jackett.IntegrationTests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <TargetFramework>net5.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <NoWarn />
     <TreatWarningsAsErrors>false</TreatWarningsAsErrors>
@@ -9,7 +9,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.8.0" />
     <PackageReference Include="MSTest.TestAdapter" Version="2.1.2" />
     <PackageReference Include="MSTest.TestFramework" Version="2.1.2" />
     <PackageReference Include="Selenium.Chrome.WebDriver" Version="85.0.0" />