Incorrect cluster setting description

**Describe the problem**

`kv.allocator.io_overload_threshold_enforce` doesn't exist:

```
kv.allocator.replica_io_overload_threshold | 0.3 | f | f | the maximum store io overload before the enforcement defined by `kv.allocator.io_overload_threshold_enforce` is taken on a store for allocation decisions | 0.3 | default
```

**To Reproduce**

`SHOW CLUSTER SETTINGS` on v25.2.0-alpha.2.

**Environment:**
- v25.2.0-alpha.2

**Additional context**

Noticed out of date description for a retired cluster setting.

Jira issue: CRDB-49165