Update target framework to support net9.0 in project files (Azure#50003)

Author: JonathanCrd

sdk/identity/Azure.Identity/integration/Integration.Identity.Common/Integration.Identity.Common.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>

sdk/identity/Azure.Identity/integration/Integration.Identity.Func/Integration.Identity.Func.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <AzureFunctionsVersion>v4</AzureFunctionsVersion>
   </PropertyGroup>
   <ItemGroup>

sdk/identity/Azure.Identity/integration/WebApp/Integration.Identity.WebApp.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
   </PropertyGroup>