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/UmbracoPackage/UmbracoPackage.csproj

@@ -1,16 +1,20 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
     <ContentTargetFolders>.</ContentTargetFolders>
-    <Product>UmbracoPackage</Product>
+    <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">UmbracoPackage</RootNamespace>
+  </PropertyGroup>
+
+  <PropertyGroup>
     <PackageId>UmbracoPackage</PackageId>
+    <Product>UmbracoPackage</Product>
     <Title>UmbracoPackage</Title>
     <Description>...</Description>
     <PackageTags>umbraco plugin package</PackageTags>
-    <RootNamespace Condition="'$(name)' != '$(name{-VALUE-FORMS-}safe_namespace)'">UmbracoPackage</RootNamespace>
-    <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
   </PropertyGroup>
-
+  
   <ItemGroup>
     <PackageReference Include="Umbraco.Cms.Web.Website" Version="UMBRACO_VERSION_FROM_TEMPLATE" />
     <PackageReference Include="Umbraco.Cms.Web.BackOffice" Version="UMBRACO_VERSION_FROM_TEMPLATE" />