Fix a few places that hardcoded the target framework to use the current .NET version instead. (#19403)

msbuild/Xamarin.HotRestart.PreBuilt/Xamarin.PreBuilt.iOS/Xamarin.PreBuilt.iOS.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0-ios</TargetFramework>
+    <TargetFramework>net$(BundledNETCoreAppTargetFrameworkVersion)-ios</TargetFramework>
     <RuntimeIdentifier>ios-arm64</RuntimeIdentifier>
     <OutputType>Exe</OutputType>
     <CustomBeforeMicrosoftCommonTargets>$(CustomBeforeMicrosoftCommonTargets);$(MSBuildThisFileDirectory)../../../tests/common/SupportedOSPlatformVersions.targets</CustomBeforeMicrosoftCommonTargets>

tools/dotnet-linker/Makefile

@@ -3,7 +3,7 @@ TOP=../..
 include $(TOP)/Make.config
 include $(TOP)/mk/rules.mk
 
-BUILD_DIR=bin/Debug/net8.0
+BUILD_DIR=bin/Debug/$(DOTNET_TFM)
 
 DOTNET_TARGETS += \
 	$(BUILD_DIR)/dotnet-linker.dll \