Update Tailwind executable call for v3.0

As of `tailwindcss-rails` `>= 3.0` the command for the executable was
extracted to a separate `tailwindcss-ruby` gem, which causes an error in
the rake tasks we generate for the `solidus_admin`.

The error this fixes is
```
NoMethodError: undefined method `executable' for Tailwindcss::Commands:Module
```

Co-authored-by: Benjamin Willems <[email protected]>

admin/lib/solidus_admin/install_tailwindcss.rb

@@ -39,7 +39,7 @@
   namespace :solidus_admin do
     namespace :tailwindcss do
       root = Rails.root
-      tailwindcss = Tailwindcss::Commands.executable
+      tailwindcss = Tailwindcss::Ruby.executable
 
       tailwindcss_command = [
         tailwindcss,