Cleanup conversion to Arcade (dotnet/extensions#1014)

* Remove obsolete targets, properties, and scripts
* Replace IsProductComponent with IsShipping
* Undo bad merge to version.props
* Update documentation, and put workarounds into a common file
* Replace usages of RepositoryRoot with RepoRoot
* Remove API baselines
* Remove unnecessary restore feeds and split workarounds into two files
* Enable PR checks on all branches, and disable autocancel


Commit migrated from dotnet/extensions@f41cfde

src/benchmarks/micro/libraries/Microsoft.Extensions.Primitives/Microsoft.Extensions.Primitives.Performance.csproj

@@ -5,7 +5,6 @@
     <OutputType>Exe</OutputType>
     <ServerGarbageCollection>true</ServerGarbageCollection>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <ItemGroup>