Ignore support_bot column in users table

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32554 removed the
support_bot column, but nodes were not told to ignore this column.
As a result, after the post-deploy migration ran, nodes attempted
to use this column, causing 500 errors.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/67295

app/models/user.rb

@@ -55,6 +55,9 @@ class User < ApplicationRecord
   BLOCKED_MESSAGE = "Your account has been blocked. Please contact your GitLab " \
                     "administrator if you think this is an error."
 
+  # Removed in GitLab 12.3. Keep until after 2019-09-22.
+  self.ignored_columns += %i[support_bot]
+
   # Override Devise::Models::Trackable#update_tracked_fields!
   # to limit database writes to at most once every hour
   # rubocop: disable CodeReuse/ServiceClass