Add and apply Rails/WhereRange rubocop rule

This rule was added in rubocop-rails 2.25.0. Applying it allows us to
simplify the code a little bit. For example, now there's no need to
specify the `proposals` table in proposal scopes, which was actually
causing a bug in the `Legislation::Proposal` model, which was using the
`proposals` table instead of the `legislation_proposals` table (but,
since we don't use this scope, it didn't affect the application).

lib/tasks/files.rake

@@ -2,9 +2,7 @@ namespace :files do
   desc "Removes cached attachments which weren't deleted for some reason"
   task remove_old_cached_attachments: :environment do
     Tenant.run_on_each do
-      ActiveStorage::Blob.unattached
-                         .where("active_storage_blobs.created_at <= ?", 1.day.ago)
-                         .find_each(&:purge_later)
+      ActiveStorage::Blob.unattached.where(created_at: ..1.day.ago).find_each(&:purge_later)
     end
   end
 end