Improve Central Package Management, update dependencies and fix package validation/MSBuild warnings (#15362)

* Use version overrides for ImageSharp 2

* Move Directory.Packages.props to root, use GlobalPackageReference and replace Microsoft.CSharp with Microsoft.CodeAnalysis.CSharp

* Replace NPoco.SqlServer with NPoco and remove unused System dependencies

* Fix package validation and MSBuild warnings

* Add nuget.config and enable package source mapping

* Remove explicitly set C# language version

* Add empty Directory.Packages.props file for acceptance test

* Downgrade SixLabors.ImageSharp back to 3.0.2 because of breaking changes

* Update ImageSharp/ImageSharp.Web to 3.1.0 and use ComputeHMAC

templates/Umbraco.Templates.csproj

@@ -9,7 +9,6 @@
     <IncludeContentInPack>true</IncludeContentInPack>
     <ContentTargetFolders>.</ContentTargetFolders>
     <NoWarn>NU5128</NoWarn>
-    <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
   </PropertyGroup>
 
   <ItemGroup>
@@ -44,7 +43,7 @@
 
   <!-- Update template.json files with the default UmbracoVersion value set to the current build version -->
   <ItemGroup>
-    <PackageReference Include="Umbraco.JsonSchema.Extensions" Version="0.3.0" PrivateAssets="all" />
+    <PackageReference Include="Umbraco.JsonSchema.Extensions" PrivateAssets="all" />
   </ItemGroup>
   <ItemGroup>
     <Content Update="**\.template.config\template.json" Pack="false" />