Add Mastodon version to user agent (#2253)

app/services/fetch_link_card_service.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class FetchLinkCardService < BaseService
-  USER_AGENT = "#{HTTP::Request::USER_AGENT} (Mastodon; +http://#{Rails.configuration.x.local_domain}/)"
+  USER_AGENT = "#{HTTP::Request::USER_AGENT} (Mastodon/#{Mastodon::VERSION}; +http://#{Rails.configuration.x.local_domain}/)"
 
   def call(status)
     # Get first http/https URL that isn't local