Bump $(ProductVersion) to 7.4.99

(Commercial) Xamarin.Android v7.4 is being tracked in the
[xamarin-android/d15-3][0] branch, which was branched from
commit 7bb6938.

Bump `$(ProductVersion)` to 7.4.99 to track development progress of
the *next* version of Xamarin.Android.

[0]: https://github.com/xamarin/xamarin-android/commits/d15-3

Author: jonpryor

Configuration.props

@@ -12,7 +12,7 @@
       Condition=" '$(DoNotLoadOSProperties)' != 'True' "
   />
   <PropertyGroup>
-    <ProductVersion>7.3.99</ProductVersion>
+    <ProductVersion>7.4.99</ProductVersion>
     <!-- *Latest* API level binding that we support; used when building src/Xamarin.Android.Build.Tasks -->
     <AndroidLatestApiLevel>25</AndroidLatestApiLevel>
     <AndroidLatestFrameworkVersion>v7.1</AndroidLatestFrameworkVersion>