Bump $(ProductVersion) to 11.1.99

(Commercial) Xamarin.Android v11.1 is being tracked in the
[xamarin-android/d16-8][0] branch, which was branched from
commit a944a89.

Bump `$(ProductVersion)` to 11.1.99 to track development progress of
the *next* version of Xamarin.Android.

[0]: https://github.com/xamarin/xamarin-android/commits/d16-8

Author: jonpryor

Directory.Build.props

@@ -13,7 +13,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <ProductVersion>11.0.99</ProductVersion>
+    <ProductVersion>11.1.99</ProductVersion>
     <!-- NuGet package version numbers. See Documentation/guides/DotNet5.md.
          Rules:
          * Reset patch version (third number) to 100 every time either major or minor version is bumped.