BUG: categorical unpickle to use _coerce_indexer_dtype
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

follow up for #13080 to use `_coerce_indexer_dtype`.

this _should_ show up in tests I think for an older pickle (I don't recall when we started coercing the codes), but not really a big deal.
 can you add a test, or do we have coverage?
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13426?src=pr) is **84.34%**

> Merging [#13426](https://codecov.io/gh/pydata/pandas/pull/13426?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13426   diff @@
==========================================
  Files           138        138          
  Lines         51126      51126          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43122      43121     -1   
- Misses         8004       8005     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [f20b41e...e8702f8](https://codecov.io/gh/pydata/pandas/compare/f20b41e73fdc84089289cd3999762892f85ec4b6...e8702f80f6c100352e23d4168bc6c81f16718aa9)
 No, added some pickles to test it.
 thanks!
