Proj0035: Remove deprecated RestoreProjectStyle property

Microsoft introduced the <RestoreProjectStyle> to smoothen package restoring using packages.config. This way of defining third-party dependencies has been deprecated for a very long time now. The property should be removed, and if the packages.config still exists its package references should be converted too.

Non-compliant

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <RestoreProjectStyle>PackageReference</RestoreProjectStyle>
  </PropertyGroup>

</Project>

Compliant

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

</Project>