[ty] Reject deleting`Final` attributes (#24508)
## Summary
We now error when attempting to delete a `Final` attribute, as in:
```python
from typing import Final
class FinalAttribute:
def __init__(self) -> None:
self.x: Final[int] = 1
final_attribute = FinalAttribute()
# error: [invalid-assignment] "Cannot delete final attribute `x` on type `FinalAttribute`"
del final_attribute.x
```
