Fix cron job base class and migration version number

- ClearOldGiteaPullRequestsJob: inherit from ApplicationJob (not CronJob)
  and use find_each(&:destroy!) to match the GitHub/GitLab pattern
- Rename aggregated migration from 1016015 to 1023015 to avoid
  collision with the recaptcha module migration

app/workers/cron/clear_old_gitea_pull_requests_job.rb

@@ -29,11 +29,12 @@
 #++
 
 module Cron
-  class ClearOldGiteaPullRequestsJob < CronJob
+  class ClearOldGiteaPullRequestsJob < ApplicationJob
-    self.cron_expression = "25 1 * * *"
+    priority_number :low
 
     def perform
-      GiteaPullRequest.without_work_package.destroy_all
+      GiteaPullRequest.without_work_package
+                      .find_each(&:destroy!)
     end
   end
 end