Add warning that gitlab-secrets isn't included

Many customers forget to include the gitlab-secrets.json file. This adds
a warning that both gitlab-secrets.json and gitlab.rb are not included
in the backup.

changelogs/unreleased/add-warning-to-backup-rake-task.yml

+---
+title: Add warning that gitlab-secrets isn't included in backup
+merge_request:
+author:
+type: other

lib/tasks/gitlab/backup.rake

@@ -20,6 +20,11 @@ namespace :gitlab do
       backup.pack
       backup.cleanup
       backup.remove_old
+
+      puts "Warning: Your gitlab.rb and gitlab-secrets.json files contain sensitive data \n" \
+           "and are not included in this backup. You will need these files to restore a backup.\n" \
+           "Please back them up manually.".color(:red)
+      puts "Backup task is done."
     end
 
     # Restore backup of GitLab system
@@ -68,6 +73,9 @@ namespace :gitlab do
       Rake::Task['cache:clear'].invoke
 
       backup.cleanup
+      puts "Warning: Your gitlab.rb and gitlab-secrets.json files contain sensitive data \n" \
+           "and are not included in this backup. You will need to restore these files manually.".color(:red)
+      puts "Restore task is done."
     end
 
     namespace :repo do