use TargetFrameworkIdentifier

this doesn't work in Directory.Build.props, moved it to Directory.Build.targets.

Directory.Build.props

@@ -28,13 +28,6 @@
     <WarningsNotAsErrors>NU1901;NU1902;NU1903;NU1904</WarningsNotAsErrors>
   </PropertyGroup>
 
-  <!--
-        Disable nullable warnings on old frameworks because of missing annotations.
-  -->
-  <PropertyGroup Condition=" !$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0')) ">
-    <NoWarn>$(NoWarn);CS8602;CS8604;CS8777</NoWarn>
-  </PropertyGroup>
-
   <!--
         Add the stylecop config to each project.
     -->

Directory.Build.targets

@@ -0,0 +1,8 @@
+<Project>
+  <!--
+    Disable nullable warnings on old frameworks because of missing annotations.
+  -->
+  <PropertyGroup Condition=" '$(TargetFrameworkIdentifier)' != '.NETCoreApp' ">
+    <NoWarn>$(NoWarn);nullable</NoWarn>
+  </PropertyGroup>
+</Project>

Renci.SshNet.sln

@@ -21,6 +21,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 		stylecop.json = stylecop.json
 		THIRD-PARTY-NOTICES.TXT = THIRD-PARTY-NOTICES.TXT
 		version.json = version.json
+		Directory.Build.targets = Directory.Build.targets
 	EndProjectSection
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{D21A4D03-0AC2-4613-BB6D-74D2D16A72CC}"

src/Renci.SshNet/Renci.SshNet.csproj

@@ -36,7 +36,7 @@
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
   </PropertyGroup>
 
-  <PropertyGroup Condition=" $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0')) ">
+  <PropertyGroup Condition=" '$(TargetFrameworkIdentifier)' == '.NETCoreApp' ">
     <IsTrimmable>true</IsTrimmable>
     <IsAotCompatible>true</IsAotCompatible>
   </PropertyGroup>