Use danger variant for repository check modal

What does this MR do and why?

For the "clear all repository checks" modal:

• Uses danger variant for modal button
• Updates modal button text

#344164 (closed) (for epic &6854 (closed))

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Enable feature flag Feature.enable(:bootstrap_confirmation_modals)
2. Navigate to http://gdk.test:3000/admin/application_settings/repository
3. Click "Clear all repository checks"
4. Observe modal.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #344164 (closed)