Ignore the application_settings.runners_registration_token column

What does this MR do and why?

The runners_registration_token_encrypted column was introduced by cbba8fe0 with the fallback: true option, which meant that it was still reading the runners_registration_token column at the time. Then with 350f91aa, the runners_registration_token attribute started to exclusively rely on the runners_registration_token_encrypted column, making the runners_registration_token unused.

I'm following the guide for dropping a column at https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#ignoring-the-column-release-m.

The target release for the actual drop is %17.6, and the removal of the ignore_column call will be done in %17.7 (the soonest is 2024-11-22).

MR acceptance checklist

Since TokenAuthenticatable redefine the reader (not the writer but it defines a set_<field> method instead), and the codebase doesn't have any runners_registration_token = value calls, ignoring/dropping the column should be safe.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.