DEPR: deprecate sortlevel in favor of sort_index
closes #14279

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15099?src=pr) is 84.75% (diff: 77.77%)
> Merging [#15099](https://codecov.io/gh/pandas-dev/pandas/pull/15099?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #15099   diff @@
==========================================
  Files           145        145          
  Lines         51220      51220          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43415      43414     -1   
- Misses         7805       7806     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [c71f214...c070ba8](https://codecov.io/gh/pandas-dev/pandas/compare/c71f214a7024a3a8d679f8c2eafdd4e870d7c216...c070ba83c9e2bfa847f19072af13c5414301c866?src=pr) updated