[dotnet] Store the .NET version we target in a generated props file. (#19416)

This way we can avoid hardcoding the .NET version later in the build targets.

dotnet/Makefile

@@ -119,6 +119,7 @@ targets/Microsoft.$(1).Sdk.Versions.props: targets/Microsoft.Sdk.Versions.templa
 		-e 's/@DOTNET_TFM@/$(DOTNET_TFM)/g' \
 		-e 's/@RUNTIME_PACK_RUNTIME_IDENTIFIERS@/$(4)/g' \
 		-e 's/@XCODE_IS_PREVIEW@/$(XCODE_IS_PREVIEW)/g' \
+		-e 's/@DOTNET_TFM@/$(DOTNET_TFM)/g' \
 		$$< > $$@.tmp
 	$$(Q) mv $$@.tmp $$@
 

dotnet/targets/Microsoft.Sdk.Versions.template.props

@@ -8,6 +8,7 @@
 		<_PackageVersion>@NUGET_VERSION_FULL@</_PackageVersion>
 		<_DefaultTargetPlatformVersion>@DEFAULT_TARGET_PLATFORM_VERSION@</_DefaultTargetPlatformVersion>
 		<_XamarinIsPreviewRelease>@XCODE_IS_PREVIEW@</_XamarinIsPreviewRelease>
+		<_XamarinDotNetVersion>@DOTNET_TFM@</_XamarinDotNetVersion>
 	</PropertyGroup>
 
 	<ItemGroup>@VALID_RUNTIME_IDENTIFIERS@

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -964,7 +964,7 @@
 			<_XamarinRefPackageDirectory>%(_XamarinFrameworkReference.TargetingPackPath)</_XamarinRefPackageDirectory>
 			<_XamarinNativeLibraryDirectory>$(_XamarinSdkRuntimePackDirectory)/runtimes/$(RuntimeIdentifier)/native</_XamarinNativeLibraryDirectory>
 			<_XamarinIncludeDirectory>$(_XamarinSdkRuntimePackDirectory)/runtimes/$(RuntimeIdentifier)/native</_XamarinIncludeDirectory>
-			<_XamarinRefAssemblyDirectory>$(_XamarinRefPackageDirectory)/ref/net8.0/</_XamarinRefAssemblyDirectory>
+			<_XamarinRefAssemblyDirectory>$(_XamarinRefPackageDirectory)/ref/$(_XamarinDotNetVersion)/</_XamarinRefAssemblyDirectory>
 			<_XamarinRefAssemblyPath>$(_XamarinRefAssemblyDirectory)$(_PlatformAssemblyName).dll</_XamarinRefAssemblyPath>
 
 			<_LibPartialStaticRegistrar Condition="'$(_XamarinRuntime)' == 'MonoVM'">$(_XamarinNativeLibraryDirectory)/Microsoft.$(_PlatformName).registrar.a</_LibPartialStaticRegistrar>