Move from netcoreapp3.1 to net6.0

I believe this is a good idea anyway, but this should assist with:

dotnet/android#7451

Which is hitting the error:

    error NU1102: Unable to find package Microsoft.NETCore.App.Host.osx-x64 with version (= 3.1.31)

.NET Core 3.1.31 is not released yet and is likely on a private feed.

Also pass `6.0.x` to the [`UseDotNet`][0] yaml task.

[0]: https://learn.microsoft.com/azure/devops/pipelines/tasks/tool/dotnet-core-tool-installer

Author: jonathanpeppers

azure-pipelines.yaml

@@ -12,7 +12,7 @@ pr:
 
 # Global variables
 variables:
-  DotNetCoreVersion: 3.1.100
+  DotNetCoreVersion: 6.0.x
 
 jobs:
 - job: build

src/Microsoft.Android.Build.BaseTasks/Microsoft.Android.Build.BaseTasks.csproj

@@ -3,7 +3,7 @@
   <Import Project="MSBuildReferences.projitems" />
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <RootNamespace>Microsoft.Android.Build.Tasks</RootNamespace>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <SignAssembly>true</SignAssembly>

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
     <LangVersion>8.0</LangVersion>
     <Nullable>enable</Nullable>
     <DefineConstants>INTERNAL_NULLABLE_ATTRIBUTES</DefineConstants>

tests/Microsoft.Android.Build.BaseTasks-Tests/Microsoft.Android.Build.BaseTasks-Tests.csproj

@@ -4,7 +4,7 @@
     <!-- $(TargetFrameworks) is used to allow the $(TargetFramework) check in Directory.Build.props to work.
         If $(TargetFramework) is declared here instead, it will not be evaluated before Directory.Build.props
         is loaded and the wrong $(TestOutputFullPath) will be used. -->
-    <TargetFrameworks>netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net6.0</TargetFrameworks>
     <RootNamespace>Microsoft.Android.Build.BaseTasks.Tests</RootNamespace>
     <IsPackable>false</IsPackable>
     <OutputPath>$(TestOutputFullPath)</OutputPath>

tests/Xamarin.Android.Tools.AndroidSdk-Tests/Xamarin.Android.Tools.AndroidSdk-Tests.csproj

@@ -4,7 +4,7 @@
     <!-- $(TargetFrameworks) is used to allow the $(TargetFramework) check in Directory.Build.props to work.
         If $(TargetFramework) is declared here instead, it will not be evaluated before Directory.Build.props
         is loaded and the wrong $(TestOutputFullPath) will be used. -->
-    <TargetFrameworks>netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net6.0</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
     <IsPackable>false</IsPackable>

tools/ls-jdks/ls-jdks.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFrameworks>net472;netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net472;net6.0</TargetFrameworks>
     <RootNamespace>Xamarin.Android.Tools</RootNamespace>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <OutputPath>$(ToolOutputFullPath)</OutputPath>