[build] Use netstandard2.0 for all workload packs (#5812)

Context: #5806
Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=4614104&view=logs&j=96fd57f5-f69e-53c7-3d47-f67e6cf9b93e&t=9492f091-fb29-57d7-1f68-ecbf86da9c24

PR #5806 (currently a draft) bumped the system mono and msbuild on
various CI machines.  As these machines are *not* VMs, and do *not*
"start from clean" for every PR build (for performance reasons), the
inadvertent result is that *every subsequent build* on these machines
now *fails* when packing our workload .nupkg files:

	/Users/builder/.dotnet/sdk/5.0.103/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(141,5):
	error NETSDK1045: The current .NET SDK does not support targeting .NET Core 6.0.
	Either target .NET Core 5.0 or lower, or use a version of the .NET SDK that supports .NET Core 6.0.
	[/Users/builder/azdo/_work/1/s/xamarin-android/build-tools/create-packs/Microsoft.Android.Sdk.proj]

The target framework used by the projects that create our workload
packs should not matter; they do not need to be `net6.0`.

Update `$(TargetFramework)` to `netstandard2.0`, which improves
compatibility when restoring, packing, or otherwise using these
projects to invoke other targets.

The `$(TargetFramework)` value *does* end up in the `.nuspec` file in
these packages however, but we can prevent this by setting the
[`$(SuppressDependenciesWhenPacking)` property][0] to true:

	<SuppressDependenciesWhenPacking>true</SuppressDependenciesWhenPacking>

Finally, these packages use the DotnetPlatform or Template PackageType
and are not intended to be functional NuGets.  We can suppress certain
nuspec related warnings to clean up build output:

	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5128: Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder do not have exact matches in the other location. Consult the list of actions below:
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5128: - Add lib or ref assemblies for the netstandard2.0 target framework
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5130: Some target frameworks declared in the dependencies group of the nuspec and the lib/ref folder have compatible matches, but not exact matches in the other location. Unless intentional, consult the list of actions below:
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5130: - Add a dependency group for net6.0 to the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: References were found in the nuspec, but some reference assemblies were not found in both the nuspec and ref folder. Add the following reference assemblies:
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add Java.Interop.dll to the net6.0 reference group in the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add Mono.Android.dll to the net6.0 reference group in the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add Mono.Android.Export.dll to the net6.0 reference group in the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add mono.android.jar to the net6.0 reference group in the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add mono.android.dex to the net6.0 reference group in the nuspec
	…\NuGet.Build.Tasks.Pack.targets(221,5): warning NU5131: - Add AndroidApiInfo.xml to the net6.0 reference group in the nuspec

[0]: https://docs.microsoft.com/nuget/reference/msbuild-targets#pack-target-inputs

Author: pjcollins

build-tools/create-packs/Directory.Build.props

@@ -2,12 +2,16 @@
   <Import Project="..\..\Directory.Build.props" />
 
   <PropertyGroup>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>netstandard2.0</TargetFramework>
     <PackageType>DotnetPlatform</PackageType>
     <Authors>Microsoft</Authors>
     <OutputPath>..\..\bin\Build$(Configuration)\nuget-unsigned\</OutputPath>
     <GenerateDependencyFile>false</GenerateDependencyFile>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
+    <!-- Remove the `<group targetFramework=".NETStandard2.0" />` entry from the .nuspec. -->
+    <SuppressDependenciesWhenPacking>true</SuppressDependenciesWhenPacking>
+    <!-- Ignore TargetFramework reference group related warnings, these are workload packs not functional NuGets. -->
+    <NoWarn>$(NoWarn);NU5128;NU5131</NoWarn>
   </PropertyGroup>
 
 </Project>

src/Microsoft.Android.Templates/Microsoft.Android.Templates.csproj

@@ -11,6 +11,8 @@
     <IncludeBuildOutput>false</IncludeBuildOutput>
     <ContentTargetFolders>content</ContentTargetFolders>
     <OutputPath>..\..\bin\Build$(Configuration)\nuget-unsigned\</OutputPath>
+    <!-- Remove the `<group targetFramework=".NETStandard2.0" />` entry from the .nuspec. -->
+    <SuppressDependenciesWhenPacking>true</SuppressDependenciesWhenPacking>
   </PropertyGroup>
 
   <Import Project="..\..\Configuration.props" />