Drop netstandard for $(NetCoreTFM)

Netstandard gives compatibility benefits for libraries to be used
against netfx or netcore frameworks.  However, for libman, all of our
projects that run on netcore already target that framework directly.

This change modifies the existing multitargeted projects to target
netcore directly instead of netstandard.  This will allow future changes
to use conditional compilation to leverage newer APIs in netcore that do
not exist in the netstandard API surface.

The only breaking change here is that the Build project now uses .NET 8
for the build task.  This means consumers of that package who are not
using .NET Framework will need to have a compatible version of the SDK;
however, this aligns with the prior change in the CLI tool to require
.NET 8 as well.

src/LibraryManager.Build/Microsoft.Web.LibraryManager.Build.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;$(NetFxTFM)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreTFM);$(NetFxTFM)</TargetFrameworks>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <PackageTags>library, package, client-side, build</PackageTags>
@@ -16,8 +16,7 @@
     <PackageReference Include="Microsoft.Build.Framework" PrivateAssets="all" />
     <PackageReference Include="Microsoft.Build.Tasks.Core" PrivateAssets="all" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" PrivateAssets="all" />
-    <PackageReference Include="System.Runtime.Loader" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
-    <Reference Include="netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51"  />
+    <PackageReference Include="System.Runtime.Loader" Condition="'$(TargetFramework)' == '$(NetCoreTFM)'" />
   </ItemGroup>
 
   <ItemGroup>

src/LibraryManager.Contracts/Microsoft.Web.LibraryManager.Contracts.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;$(NetFxTFM)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreTFM);$(NetFxTFM)</TargetFrameworks>
     <PackageTags>library, package, client-side</PackageTags>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>

src/LibraryManager/Microsoft.Web.LibraryManager.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;$(NetFxTFM)</TargetFrameworks>
+    <TargetFrameworks>$(NetCoreTFM);$(NetFxTFM)</TargetFrameworks>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <PackageTags>library, package, client-side</PackageTags>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -10,11 +10,11 @@
     <PackageReference Include="Minimatch" NoWarn="NU1603" />
     <PackageReference Include="Newtonsoft.Json" />
     <PackageReference Include="System.Security.Cryptography.ProtectedData" />
-    <PackageReference Include="System.ValueTuple" Condition="'$(TargetFramework)' != 'netstandard2.0'" />
+    <PackageReference Include="System.ValueTuple" Condition="'$(TargetFramework)' != '$(NetCoreTFM)'" />
     <ProjectReference Include="..\LibraryManager.Contracts\Microsoft.Web.LibraryManager.Contracts.csproj" />
-    <Reference Include="System.ComponentModel.Composition" Condition="'$(TargetFramework)' != 'netstandard2.0'" />
-    <Reference Include="System.Net.Http" Condition="'$(TargetFramework)' != 'netstandard2.0'" />
-    <Reference Include="System.Web" Condition="'$(TargetFramework)' != 'netstandard2.0'" />
+    <Reference Include="System.ComponentModel.Composition" Condition="'$(TargetFramework)' != '$(NetCoreTFM)'" />
+    <Reference Include="System.Net.Http" Condition="'$(TargetFramework)' != '$(NetCoreTFM)'" />
+    <Reference Include="System.Web" Condition="'$(TargetFramework)' != '$(NetCoreTFM)'" />
   </ItemGroup>
   <ItemGroup>
     <Compile Update="Resources\Text.Designer.cs">