fix build warning-as-error which is suddenly appearing on net6.0

IsAotCompatible not supported on net6.0

Author: Rob-Hague

src/Renci.SshNet/Renci.SshNet.csproj

@@ -31,6 +31,10 @@
   </PropertyGroup>
 
   <PropertyGroup Condition=" $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net6.0')) ">
+    <IsTrimmable>true</IsTrimmable>
+  </PropertyGroup>
+  
+  <PropertyGroup Condition=" $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net8.0')) ">
     <IsAotCompatible>true</IsAotCompatible>
   </PropertyGroup>