Resolve "Delete Rotation"

What does this MR do?

A tiny MVC for the frontend portion of #262863 (closed) where we add the frontend code and supporting specs for the deletion modal which is currently hooked up to mock data i.e. mocks/mock_rotation.json.

🇺🇦 This feature is behind the feature flag oncall_schedules_mvc and the minimal license is GitLab Premium 👀

To check Enable the feature flag:

• Navigate to Operations => On-call Schedules or direct link is your_project/-/oncall_schedules e.g. http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/oncall_schedules.
• Click "Add schedule on empty state"
• Add a base schedule
• You will see a new grid pre-populated with a set of fake rotations
• Use the delete rotation button on the left side of any rotation to see the modal
• Clicking cancel should hide the modal and reset the rotationToUpdate attribute
• Clicking Delete should throw an GraphQL error visible inside the modal header for the moment

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #262863 (closed)