[build] Update stable .NET to 9.

Directory.Build.props

@@ -6,7 +6,7 @@
     <DotNetTargetFramework>net$(DotNetTargetFrameworkVersion)</DotNetTargetFramework>
     <DotNetAndroidTargetFramework>$(DotNetTargetFramework)-android</DotNetAndroidTargetFramework>
     <!-- Used for bootstrap, command-line tooling, and desktop NUnit projects -->
-    <DotNetStableTargetFramework>net8.0</DotNetStableTargetFramework>
+    <DotNetStableTargetFramework>net9.0</DotNetStableTargetFramework>
     <TargetFrameworkNETStandard>netstandard2.0</TargetFrameworkNETStandard>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
     <BuildOutputDirectory>$(MSBuildThisFileDirectory)bin\$(Configuration)\</BuildOutputDirectory>

Makefile

@@ -7,7 +7,7 @@ RUNTIME       := $(shell which mono64 2> /dev/null && echo mono64 || echo mono)
 SOLUTION       = Xamarin.Android.sln
 TEST_TARGETS   = build-tools/scripts/RunTests.targets
 API_LEVEL     ?=
-PREPARE_NET_FX = net8.0
+PREPARE_NET_FX = net9.0
 PREPARE_ARGS =
 PREPARE_PROJECT = build-tools/xaprepare/xaprepare/xaprepare.csproj
 PREPARE_MSBUILD_FLAGS = $(PREPARE_MSBUILD_ARGS) $(MSBUILD_ARGS)

build-tools/automation/yaml-templates/variables.yaml

@@ -24,7 +24,7 @@ variables:
 - name: NUnit.NumberOfTestWorkers
   value: 4
 - name: DotNetSdkVersion
-  value: 8.0
+  value: 9.0
 - name: DotNetSdkQuality
   value: GA
 - name: GitHub.Token
@@ -50,7 +50,7 @@ variables:
 - name: DotNetTargetFramework
   value: net9.0
 - name: DotNetStableTargetFramework
-  value: net8.0
+  value: net9.0
 # Workaround: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1585820
 - name: _WriteTelemetryProperties
   value: false