remove government/admin from block manager url

this is not relevant to the Content Block Manager,
so we are just having the endpoint be
`/content-block-manager`.

config/routes.rb

@@ -459,5 +459,5 @@ def redirect(path, options = { prefix: Whitehall.router_prefix })
 
   mount GovukPublishingComponents::Engine, at: "/component-guide" unless Rails.env.production?
 
-  mount ContentBlockManager::Engine, at: "government/admin/content-block-manager", as: "content_block_manager"
+  mount ContentBlockManager::Engine, at: "content-block-manager", as: "content_block_manager"
 end