V2: restricts deletion of segments that are currently in use

### Bug Description

Flipt v2 should not allow to delete such segments to prevent unintended inconsistent state.

### Which major version?

v2 (Alpha)

### Version Info

v2-alpha

### Search

- [x] I searched for other open and closed issues before opening this

### Steps to Reproduce

-

### Expected Behavior

_No response_

### Additional Context

_No response_