build: retarget from netstandard2.0 to net472 and update dependencies

Author: onyxmaster

StructuredFieldValues/StructuredFieldValues.csproj

@@ -2,14 +2,14 @@
   <Import Project="..\shared\Minimal.props" />
 
   <PropertyGroup>
-    <TargetFrameworks>$(LibraryTargetFramework);$(ApplicationTargetFramework)</TargetFrameworks>
+    <TargetFrameworks>$(LegacyTargetFramework);$(ApplicationTargetFramework)</TargetFrameworks>
     <Nullable>enable</Nullable>
     <Version>0.6.4</Version>
     <Description>A parser for RFC 8941 Structured Field Values for HTTP.</Description>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Memory" Version="4.6.0" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
+    <PackageReference Include="Microsoft.Bcl.Memory" Version="9.0.6" Condition="!$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'netstandard2.1'))" />
   </ItemGroup>
 
   <PropertyGroup Label="PackageSettings">