Resolve "Replace `data-confirm` modals with `GlModal` in `app/views/projects/pipeline_schedules/_pipeline_schedule.html.haml`"

What does this MR do and why?

Update the confirmation button variant to danger and its label to Delete on delete pipeline schedule confirmation modal.

Screenshots or screen recordings

Without FF	With FF (before)	With FF (after)

How to set up and validate locally

1. Enable the bootstrap_confirmation_modals feature flag: Feature.enable(:bootstrap_confirmation_modals)
2. Go to a project > CI/CD > Schedules
3. If there isn't a schedule, create one by clicking New schedule
4. Click Delete icon next to one of the schedules on the list

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 #344180 (closed)