Bump $(ProductVersion) to 12.0.99

Context: https://developer.android.com/about/versions/12
Context: dotnet#6234

We (try to) base Xamarin.Android product versions on the latest
Android versions, and API-31 is "Android 12".  With the merging of
stable API-31 support (5f408eb, e722825, others), we want our first
release with API-30 support to likewise be v12.0.

The [d16-11][0] branch is planned on being the first release which
will offer stable API-31 support, via PR dotnet#6234, and once merged the
d16-11 branch will report itself as v12.0.

As xamarin-android/main is tracking the *next* Xamarin.Android
version, bump `$(ProductVersion)` to v12.0.99 for eventual release
as v12.1.0.

[0]: https://github.com/xamarin/xamarin-android/commits/d16-11

Directory.Build.props

@@ -13,7 +13,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <ProductVersion>11.4.99</ProductVersion>
+    <ProductVersion>12.0.99</ProductVersion>
     <!-- NuGet package version numbers. See Documentation/guides/OneDotNet.md.
          Rules:
          * Major/Minor match Android stable API level, such as 30.0 for API 30.