Make `unnecessary-paren-on-raise-exception` an unsafe edit (#8231)
## Summary
This rule is now unsafe if we can't verify that the `obj` in `raise
obj()` is a class or builtin. (If we verify that it's a function, we
don't raise at all, as before.)
See the documentation change for motivation behind the unsafe edit.
Closes https://github.com/astral-sh/ruff/issues/8228.
