replace change_table with add_column in migration

I was running into a `TypeError: can't quote Array (TypeError)`
error when running this migration and at first I thought using
`change_table` instead of `add_column` seemed to help.

In actuality the error still occurs when this migration is run as part
of a sequence of multiple migrations, but the migration runs
successfully when run on its own.

However, the issue seems to be fixed in Rails 7.1 so:

- can switch this to use `add_column` which is clearer in terms of the
  intent, and
- wait until our Rails 7.1 upgrade is merged before shipping this

Author: mikej

db/migrate/20250820145012_add_dietary_restrictions_to_members.rb

@@ -1,8 +1,6 @@
 class AddDietaryRestrictionsToMembers < ActiveRecord::Migration[7.0]
   def change
-    change_table :members do |t|
-      t.enum :dietary_restrictions, enum_type: "dietary_restriction_enum", array: true, default: []
-    end
+    add_column :members, :dietary_restrictions, :enum, enum_type: "dietary_restriction_enum", array: true, default: []
     add_column :members, :other_dietary_restrictions, :string
   end
 end