$(AndroidPackVersionSuffix)=preview.7; net9 is 34.99.0.preview.7 (#9052)

Context: https://github.com/xamarin/xamarin-android/tree/release/9.0.1xx-preview6

We branched for .NET 9 Preview 6 from ddb215b into release/9.0.1xx-preview6;
the main branch is now .NET 9 Preview 7.

Update xamarin-android/main's version number to 34.99.0-preview.7.

Author: jonathanpeppers

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.6</AndroidPackVersionSuffix>
+    <AndroidPackVersionSuffix>preview.7</AndroidPackVersionSuffix>
   </PropertyGroup>
 
   <!-- Common <PackageReference/> versions -->