Show confirmation toast on milestone deletion

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Summary

Originally raised on Slack: "I've noticed that when deleting a milestone, whether from the listing page or the detail page, we don't provide a contextual message based on the user's action. Is this a known issue?"

When deleting a milestone the user is prompted to confirm the deletion, but is not shown a success message after the deletion. While users can see that the milestone is removed from the list, showing a toast will reinforce that the deletion succeeded.

Acceptance criteria

  • When a milestone is successfully deleted a toast message appears stating "Milestone deleted."
Edited by 🤖 GitLab Bot 🤖