[Microsoft.Android.Sdk.ILLink] target net7.0 temporarily (#7803)

Context: dotnet/macios#17560
Context: dotnet/runtime#82241

On macOS, if you have both .NET 7 and .NET 8 Preview 1 installed, and
you do:

	dotnet new android
	dotnet build -c Release

You get a build error:

	Fatal error in IL Linker
	Unhandled exception. System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
	---> System.IO.FileNotFoundException: Could not load file or assembly 'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.
	File name: 'System.Runtime, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
	    at Microsoft.Android.Sdk.ILLink.PreserveSubStepDispatcher..ctor()
	    at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
	    --- End of inner exception stack trace ---
	    at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean wrapExceptions)
	    at Mono.Linker.Driver.AddCustomStep(Pipeline pipeline, String arg)
	    at Mono.Linker.Driver.SetupContext(ILogger customLogger)
	    at Mono.Linker.Driver.Run(ILogger customLogger)
	    at Mono.Linker.Driver.Main(String[] args)

This happens because the .NET 8 linker is actually trying to run
against the .NET 7 runtime!  It promptly blows up against `net8.0`
linker steps.

Trying [the same workaround as xamarin-macios][0], to build our
linker steps for `net7.0`.

[0]: dotnet/macios@c61e327

Author: jonathanpeppers

src/Microsoft.Android.Sdk.ILLink/Microsoft.Android.Sdk.ILLink.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <Import Project="..\..\Configuration.props" />
   <PropertyGroup>
-    <TargetFramework>$(DotNetTargetFramework)</TargetFramework>
+    <TargetFramework>$(DotNetStableTargetFramework)</TargetFramework>
     <DefineConstants>ILLINK</DefineConstants>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <OutputPath>$(MicrosoftAndroidSdkOutDir)</OutputPath>