Rename the I18n keys for associations' restrict_dependent_destroy errors

The `:one` and `:many` keys are used for pluralisations, but as such are incorrect in english (since we should be using `:one` for singular and `:other` for plurals).

However in this particular case, we're not describing a pluralized entity, we're describing an error for a particular type of association.

This commit changes the I18n key to be the type of association rather than a misleading pluralization key, so that apps that test the validity of their pluralizations can continue to work.