[build] update $(MSBuildPackageReferenceVersion) to 17.6.3 (#221)

Context: dotnet/android#8366

dotnet/android#8366 is attempting to build xamarin-android
against .NET 9 previews, and currently fails:

	(Restore target) ->
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605: Warning As Error: Detected package downgrade: Microsoft.Build.Framework from 17.5.0 to 17.3.2. Reference the package directly from the project to select a different version.
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605:  MSBuildDeviceIntegration -> MSBuild.StructuredLogger 2.2.100 -> doh (>= 17.5.0)
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605:  MSBuildDeviceIntegration -> Microsoft.Build.Framework (>= 17.3.2)
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605: Warning As Error: Detected package downgrade: Microsoft.Build.Utilities.Core from 17.5.0 to 17.3.2. Reference the package directly from the project to select a different version.
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605:  MSBuildDeviceIntegration -> MSBuild.StructuredLogger 2.2.100 -> Microsoft.Build.Utilities.Core (>= 17.5.0)
	tests/MSBuildDeviceIntegration/MSBuildDeviceIntegration.csproj : error NU1605:  MSBuildDeviceIntegration -> Microsoft.Build.Utilities.Core (>= 17.3.2)

We need to update `MSBuild.StructuredLogger` package to be able to
move to .NET 9, as the `.binlog` file format changed in .NET 9.
The newer `MSBuild.StructuredLogger` package, in turn, requires a
newer `Microsoft.Build.Framework` package version, which is
controlled here.

Update the `$(MSBuildPackageReferenceVersion)` MSBuild property so
that (1) it can now be overridden, which would allow version updates
in the future without requiring submodule updates, and (2) update the
the default value to 17.6.3.  I fear that if we move to 17.8.3, we
may potentially break other repos.

VS 2022 17.6 is an LTS release that feels "safe", is new enough to solve
the issue, and hopefully won't break anyone?

Additionally, update the `System.Security.Cryptography.Xml` package
so that it now uses an overridable
`$(SystemSecurityCryptographyXmlVersion)` MSBuild property, which
defaults to 7.0.1, in order to fix this build error:

	src\Microsoft.Android.Build.BaseTasks\Microsoft.Android.Build.BaseTasks.csproj : error NU1605: Detected package downgrade: System.Security.Cryptography.Xml from 7.0.1 to 6.0.1. Reference the package directly from the project to select a different version.
	src\Microsoft.Android.Build.BaseTasks\Microsoft.Android.Build.BaseTasks.csproj : error NU1605:  Microsoft.Android.Build.BaseTasks -> Microsoft.Build.Tasks.Core 17.6.3 -> System.Security.Cryptography.Xml (>= 7.0.1)
	src\Microsoft.Android.Build.BaseTasks\Microsoft.Android.Build.BaseTasks.csproj : error NU1605:  Microsoft.Android.Build.BaseTasks -> System.Security.Cryptography.Xml (>= 6.0.1)
	tests\Microsoft.Android.Build.BaseTasks-Tests\Microsoft.Android.Build.BaseTasks-Tests.csproj : error NU1605: Detected package downgrade: System.Security.Cryptography.Xml from 7.0.1 to 6.0.1. Reference the package directly from the project to select a different version.
	tests\Microsoft.Android.Build.BaseTasks-Tests\Microsoft.Android.Build.BaseTasks-Tests.csproj : error NU1605:  Microsoft.Android.Build.BaseTasks-Tests -> Microsoft.Build.Tasks.Core 17.6.3 -> System.Security.Cryptography.Xml (>= 7.0.1)
	tests\Microsoft.Android.Build.BaseTasks-Tests\Microsoft.Android.Build.BaseTasks-Tests.csproj : error NU1605:  Microsoft.Android.Build.BaseTasks-Tests -> System.Security.Cryptography.Xml (>= 6.0.1)

src/Microsoft.Android.Build.BaseTasks/MSBuildReferences.projitems

@@ -4,7 +4,8 @@
 <Project>
   <!--Import this file in projects needing to reference Microsoft.Build.*.dll -->
   <PropertyGroup>
-    <MSBuildPackageReferenceVersion>17.3.2</MSBuildPackageReferenceVersion>
+    <MSBuildPackageReferenceVersion Condition=" '$(MSBuildPackageReferenceVersion)' == '' ">17.8.3</MSBuildPackageReferenceVersion>
+    <SystemSecurityCryptographyXmlVersion Condition=" '$(SystemSecurityCryptographyXmlVersion)' == '' ">7.0.1</SystemSecurityCryptographyXmlVersion>
     <LibZipSharpVersion Condition=" '$(LibZipSharpVersion)' == '' " >3.0.0</LibZipSharpVersion>
     <MonoUnixVersion>7.1.0-final.1.21458.1</MonoUnixVersion>
   </PropertyGroup>
@@ -14,7 +15,7 @@
     <PackageReference Include="Microsoft.Build.Framework"      Version="$(MSBuildPackageReferenceVersion)" />
     <PackageReference Include="Microsoft.Build.Tasks.Core"     Version="$(MSBuildPackageReferenceVersion)" />
     <PackageReference Include="Microsoft.Build.Utilities.Core" Version="$(MSBuildPackageReferenceVersion)" />
-    <PackageReference Include="System.Security.Cryptography.Xml" Version="6.0.1" />
+    <PackageReference Include="System.Security.Cryptography.Xml" Version="$(SystemSecurityCryptographyXmlVersion)" />
     <PackageReference Include="K4os.Compression.LZ4" Version="1.1.11" />
     <PackageReference Include="Xamarin.Build.AsyncTask" Version="0.4.0" GeneratePathProperty="true" />
     <PackageReference Include="Xamarin.LibZipSharp" Version="$(LibZipSharpVersion)" GeneratePathProperty="true" />