Opt out of Rails/UnusedIgnoredColumns cop (#28385)

.rubocop.yml

@@ -109,6 +109,12 @@ Rails/SkipsModelValidations:
   Exclude:
     - 'db/*migrate/**/*'
 
+# Reason: We want to preserve the ability to migrate from arbitrary old versions,
+# and cannot guarantee that every installation has run every migration as they upgrade.
+# https://docs.rubocop.org/rubocop-rails/cops_rails.html#railsunusedignoredcolumns
+Rails/UnusedIgnoredColumns:
+  Enabled: false
+
 # Reason: Some single letter camel case files shouldn't be split
 # https://docs.rubocop.org/rubocop-rspec/cops_rspec.html#rspecfilepath
 RSpec/FilePath: