API: fix str-accessor on CategoricalIndex
closes #23555
closes #23556

split off from #23167
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry


Hello @h-vetinari! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/strings.py`](https://github.com/h-vetinari/pandas/blob/98ff497164c431abc753c1384f076b1aeb817c45/pandas/core/strings.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_strings.py`](https://github.com/h-vetinari/pandas/blob/98ff497164c431abc753c1384f076b1aeb817c45/pandas/tests/test_strings.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=h1) Report
> Merging [#24005](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92d25f0da6c3b1175047cba8c900e04da68920b8?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `50%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24005/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24005   +/-   ##
=======================================
  Coverage   42.45%   42.45%           
=======================================
  Files         161      161           
  Lines       51561    51561           
=======================================
  Hits        21888    21888           
  Misses      29673    29673
```

| Flag | Coverage Δ | |
|---|---|---|
| #single | `42.45% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/24005/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `33% <50%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=footer). Last update [92d25f0...ecd08b9](https://codecov.io/gh/pandas-dev/pandas/pull/24005?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback 
Ping on green thanks @h-vetinari 