Bump $(ProductVersion) to 7.3.99

(Commercial) Xamarin.Android v7.3 is being tracked in the
[xamarin-android/d15-2][0] branch, which was branched from
commit 583ed5f.

Bump `$(ProductVersion)` to 7.3.99 to track development progress of
the *next* version of Xamarin.Android.

[0]: https://github.com/xamarin/xamarin-android/commits/d15-2

Configuration.props

@@ -12,7 +12,7 @@
       Condition=" '$(DoNotLoadOSProperties)' != 'True' "
   />
   <PropertyGroup>
-    <ProductVersion>7.2.99</ProductVersion>
+    <ProductVersion>7.3.99</ProductVersion>
     <AutoProvision Condition=" '$(AutoProvision)' == '' ">False</AutoProvision>
     <AutoProvisionUsesSudo Condition=" '$(AutoProvisionUsesSudo)' == '' ">False</AutoProvisionUsesSudo>
     <HostOS Condition=" '$(HostOS)' == '' And '$(OS)' == 'Windows_NT' ">Windows</HostOS>