Aspire CLI global tool target net8 (#12454)

Author: eerhardt

src/Aspire.Cli/Aspire.Cli.csproj

@@ -19,6 +19,7 @@
 
   <!-- Set the nuget properties when building as a global tool. -->
   <PropertyGroup Condition="'$(IsCliToolProject)' == 'true'">
+    <TargetFramework>$(DefaultTargetFramework)</TargetFramework>
     <IsPackable>true</IsPackable>
     <PackAsTool>true</PackAsTool>
     <ToolCommandName>aspire</ToolCommandName>

src/Aspire.Cli/Backchannel/ExtensionBackchannel.cs

@@ -707,6 +707,10 @@ private X509Certificate2 GetCertificate()
         var serverCertificate = _configuration[KnownConfigNames.ExtensionCert];
         Debug.Assert(!string.IsNullOrEmpty(serverCertificate));
         var data = Convert.FromBase64String(serverCertificate);
+#if NET9_0_OR_GREATER
         return X509CertificateLoader.LoadCertificate(data);
+#else
+        return new X509Certificate2(data);
+#endif
     }
 }