Add inline validation to admin deletion protection settings

Problem to solve

@rob.hunt commented in !86568 (merged):

question: Since this is now a Vue app, have we considered adding inline feedback as well as the submit prevention? 🤔 It would be more inline with our design guidelines and we are usually only limited due to HAML.

I had a go but I wasn't able to get it to sit below the input in the short amount of time I spent on it 😅

Implementation plan

1. Add inline validation to ee/app/assets/javascripts/admin/application_settings/deletion_protection/components/form_group.vue.
2. Update specs.