$(AndroidPackVersionSuffix)=preview.6; net9 is 34.99.0.preview.6 (#8989)

Context: c611743
Context: https://github.com/xamarin/xamarin-android/tree/release/9.0.1xx-preview5

We branched for .NET 9 Preview 5 from c611743 into release/9.0.1xx-preview5;
the main branch is now .NET 9 Preview 6.

Update xamarin-android/main's version number to 34.99.0-preview.6.

Directory.Build.props

@@ -36,7 +36,7 @@
          * Bump first digit of the patch version for feature releases (and reset the first two digits to 0)
     -->
     <AndroidPackVersion>34.99.0</AndroidPackVersion>
-    <AndroidPackVersionSuffix>preview.5</AndroidPackVersionSuffix>
+    <AndroidPackVersionSuffix>preview.6</AndroidPackVersionSuffix>
   </PropertyGroup>
 
   <!-- Common <PackageReference/> versions -->