Bump LibZipSharp to 2.0.0-alpha6

The new version switched back to `zlib` from `zlib-ng` and increased the
compression level it uses to 9 (from the previous 6)

Author: grendello

src/Microsoft.Android.Build.BaseTasks/MSBuildReferences.projitems

@@ -6,7 +6,7 @@
   <PropertyGroup>
     <MSBuildReferenceVersion>15.1.0.0</MSBuildReferenceVersion>
     <MSBuildPackageReferenceVersion>16.5</MSBuildPackageReferenceVersion>
-    <LibZipSharpVersion>2.0.0-alpha4</LibZipSharpVersion>
+    <LibZipSharpVersion>2.0.0-alpha6</LibZipSharpVersion>
     <MonoUnixVersion>7.0.0-alpha8.21302.6</MonoUnixVersion>
   </PropertyGroup>