[`CountryEnumable`] Migrate to storing alpha2 and drop `country_enum_list`

At the moment, our `CountryEnumable` uses both the `countries` gem (`ISO3166::Country`) in addition to our own custom-defined hash located in [`CountryEnumable::country_enum_list`](https://github.com/hackclub/bank/blob/cd3adb8039c58e491c356bdef03f2b3dfaa7cef9/app/models/concerns/country_enumable.rb#L22)

The goal is to drop our custom-defined `CountryEnumable::country_enum_list` and solely depend on the `countries` gem for our list of countries. To make this happen, we need to migrate from storing a country as an integer to storing it as alpha2.

Start with app/models/concerns/country_enumable.rb and inspect closed pull request #9499 for prior context. Trace how CountryEnumable values are persisted and consumed, then verify that the custom country_enum_list is no longer needed and countries are stored using alpha2 values.

Written by the indexing model from the issue text.