Upgrade guide .net framework 4.5 to 4.8

Hello! 

We are in the process of upgrading .net framework version from 4.5 to 4.8 for a legacy application.

We have class library projects, web forms project and an MVC project.

1.

The system.* assemblies for some of the projects in the the .csproject file look like below. I don't think these are installed as nuget packages. instead, i believe these are added as reference from the GAC. is this assumption correct? 

If so, as part of the upgrade process, should i remove these assembly references and install them as nuget packages instead or shall i keep the assemblies as they are? 

<Reference Include="System" />
<Reference Include="System.Data" />
<Reference Include="System.Data.DataSetExtensions" />
<Reference Include="System.Data.Linq">
  <RequiredTargetFramework>3.5</RequiredTargetFramework>
</Reference>
<Reference Include="System.Web.ApplicationServices" />
<Reference Include="System.Web.DynamicData" />
<Reference Include="System.Web.Entity" />
<Reference Include="System.Drawing" />
<Reference Include="System.Web" />
<Reference Include="System.Web.Extensions" />
<Reference Include="System.Xml" />
<Reference Include="System.Configuration" />
<Reference Include="System.Web.Services" />
<Reference Include="System.EnterpriseServices" />
<Reference Include="System.Web.Mobile" />
<Reference Include="System.Xml.Linq" />

2.

How do i determine should i need to use a newer version of the assemblies than the current ones while upgrading to framework 4.8?

3.

Here is the collective list of all the System.* assemblies that are referenced in the projects in the solution

System
System.Core
System.Xml.Linq
System.Data.DataSetExtensions
System.Data
System.Xml
System.configuration
System.Data.Linq
System.Web
System.Web.ApplicationServices
System.Web.DynamicData
System.Web.Entity
System.Drawing
System.Web.Extensions
System.Web.Services
System.EnterpriseServices
System.Web.Mobile