fix System.Collections.Immutable version

Author: SimonCropp

src/Directory.Build.props

@@ -11,7 +11,6 @@
     <ResolveAssemblyReferencesSilent>true</ResolveAssemblyReferencesSilent>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
-    <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
   </PropertyGroup>
   <ItemGroup>
     <Using Include="System.ReadOnlySpan&lt;System.Char&gt;" Alias="CharSpan" />

src/Directory.Packages.props

@@ -10,7 +10,8 @@
     <PackageVersion Include="NUnit3TestAdapter" Version="4.6.0" />
     <PackageVersion Include="Polyfill" Version="7.11.0" />
     <PackageVersion Include="ProjectDefaults" Version="1.0.147" />
-    <PackageVersion Include="System.Collections.Immutable" Version="9.0.0" />
+    <!-- Keep at 8 to support old frameworks -->
+    <PackageVersion Include="System.Collections.Immutable" Version="8.0.0" />
     <PackageVersion Include="System.Memory" Version="4.6.0" />
   </ItemGroup>
 </Project>