Fix NET6_0_OR_GREATER msbuild check

build/Base.props

@@ -1,5 +1,6 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <ItemGroup Condition="'$(TargetFramework)' != 'net6'">
+  <!-- '!NET6_0_OR_GREATER' equivalent -->
+  <ItemGroup Condition="!('$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0')))">
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
     <PackageReference Include="Microsoft.Bcl.AsyncInterfaces" Version="6.0.0" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />

build/System.Memory.props

@@ -1,5 +1,6 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
-  <ItemGroup Condition="'$(TargetFramework)' != 'net6'">
+  <!-- '!NET6_0_OR_GREATER' equivalent -->
+  <ItemGroup Condition="!('$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0')))">
     <PackageReference Include="System.Memory" Version="4.5.3" />
   </ItemGroup>
 </Project>