Improve delete webhook modal copy

What does this MR do and why?

Related to #344192 (closed), towards epic &6854 (closed)

This MR validates that GlModal is being used correctly as per epic, and also updates the copy to be more meaningful.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the bootstrap_confirmation_modals feature flag

   echo "Feature.enable(:bootstrap_confirmation_modals)" | rails c

2. On a GitLab project, using the left sidebar, navigate to Settings > Webhooks

3. Create a new webhook (any random URL is fine)

4. In the Project Hooks table below the form, click the delete button for the webhook.

5. Observe the 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 #344192 (closed)