Bump to NuGet/NuGet.Client@aa7eb998 (#9682)

Fixes: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/2284231

Changes: NuGet/NuGet.Client@324d727...aa7eb99

	% git diff --shortstat 324d7272...aa7eb998
	 1057 files changed, 38053 insertions(+), 16423 deletions(-)

Author: jonpryor

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

@@ -28,7 +28,7 @@
   <ItemGroup>
     <PackageReference Include="Mono.Cecil" Version="$(MonoCecilVersion)" GeneratePathProperty="true" />
     <PackageReference Include="Irony" />
-    <PackageReference Include="NuGet.ProjectModel" Version="6.11.0" />
+    <PackageReference Include="NuGet.ProjectModel" Version="6.12.1" />
     <PackageReference Include="Newtonsoft.Json" Version="$(NewtonsoftJsonPackageVersion)" />
     <PackageReference Include="System.CodeDom" />
     <PackageReference Include="System.IO.Hashing" Version="$(SystemIOHashingPackageVersion)" />