Update more NuGet package versions

*Windows* smoke tests crash ~instantly:

https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=6930346&view=ms.vss-test-web.build-test-results-tab&runId=60074246&resultId=100047&paneView=debug

```
System.TypeInitializationException : The type initializer for 'Xamarin.ProjectTools.XamarinAndroidCommonProject' threw an exception.
----> System.TypeInitializationException : The type initializer for 'SixLabors.ImageSharp.Configuration' threw an exception.
----> System.IO.FileLoadException : Could not load file or assembly 'System.Memory, Version=4.0.1.1, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
   at Xamarin.ProjectTools.XamarinAndroidCommonProject..ctor(String debugConfigurationName, String releaseConfigurationName)
   at Xamarin.ProjectTools.XamarinAndroidApplicationProject..ctor(String debugConfigurationName, String releaseConfigurationName, String packageName) in C:\a\_work\1\s\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.ProjectTools\Android\XamarinAndroidApplicationProject.cs:line 41
   at Xamarin.Android.Build.Tests.PackagingTest.CheckSignApk(Boolean useApkSigner, Boolean perAbiApk) in C:\a\_work\1\s\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.Android.Build.Tests\PackagingTest.cs:line 381
--TypeInitializationException
   at SixLabors.ImageSharp.Image.Load(Stream stream, IImageFormat& format)
   at Xamarin.ProjectTools.XamarinAndroidCommonProject..cctor() in C:\a\_work\1\s\src\Xamarin.Android.Build.Tasks\Tests\Xamarin.ProjectTools\Android\XamarinAndroidCommonProject.cs:line 40
--FileLoadException
   at SixLabors.ImageSharp.Configuration..ctor(IConfigurationModule[] configurationModules)
   at SixLabors.ImageSharp.Configuration.CreateDefaultInstance()
   at System.Lazy`1.CreateValue()
   at System.Lazy`1.LazyInitValue()
   at SixLabors.ImageSharp.Configuration..cctor()
```

Not sure what exactly broke it, but we need to try to fix it.

Complicating the "fix" side of things, I can't build `xabuild.exe`
on Windows right now, so I'm flying blind here.

Update System.Collections.Immutable to 7.0.0 (latest; why not?).

Update SixLabors.ImageSharp to 2.1.3.

I hope this is the actual fix, because `SixLabors.ImageSharp.dll`
v2.1.3 no longer *references* `System.Memory.dll`, so failure to
find the assembly should be fine!  (Right?  Right?!)

Author: jonpryor

Directory.Build.props

@@ -47,7 +47,7 @@
     <NuGetApiPackageVersion>5.4.0</NuGetApiPackageVersion>
     <LZ4PackageVersion>1.1.11</LZ4PackageVersion>
     <MonoOptionsVersion>6.12.0.148</MonoOptionsVersion>
-    <SystemCollectionsImmutableVersion>6.0.0</SystemCollectionsImmutableVersion>
+    <SystemCollectionsImmutableVersion>7.0.0</SystemCollectionsImmutableVersion>
     <SystemRuntimeCompilerServicesUnsafeVersion>6.0.0</SystemRuntimeCompilerServicesUnsafeVersion>
     <ELFSharpVersion>2.13.1</ELFSharpVersion>
     <MdocPackageVersion Condition=" '$(MdocPackageVersion)' == '' ">5.8.9.2</MdocPackageVersion>

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Xamarin.ProjectTools.csproj

@@ -25,7 +25,7 @@
     </Content>
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="SixLabors.ImageSharp" Version="1.0.0" />
+    <PackageReference Include="SixLabors.ImageSharp" Version="2.1.3" />
     <PackageReference Include="Microsoft.Win32.Registry" />
     <ProjectReference Include="..\..\..\..\external\xamarin-android-tools\src\Xamarin.Android.Tools.AndroidSdk\Xamarin.Android.Tools.AndroidSdk.csproj">
       <Project>{E34BCFA0-CAA4-412C-AA1C-75DB8D67D157}</Project>

tools/xabuild/App.config

@@ -39,7 +39,7 @@
     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
       <dependentAssembly>
         <assemblyIdentity name="System.Collections.Immutable" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
-        <bindingRedirect oldVersion="0.0.0.0-99.9.9.9" newVersion="6.0.0.0" />
+        <bindingRedirect oldVersion="0.0.0.0-99.9.9.9" newVersion="7.0.0.0" />
       </dependentAssembly>
     </assemblyBinding>
     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">