CLN: use 'codes' rather than 'values' internally in Categorical
In some places a variable has the name ``values``, where ``codes`` would be more logical, as we're dealing with codes.

This makes it a bit easier to understand what we're looking at when reading IMO,
+1 on moving away from ‘values’, too many variants of that exist at the moment # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=h1) Report
> Merging [#22547](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1a798be98b7a605693c6d1143ab1d703a77f47a1?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22547/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22547      +/-   ##
==========================================
- Coverage   92.04%   92.04%   -0.01%     
==========================================
  Files         169      169              
  Lines       50783    50782       -1     
==========================================
- Hits        46741    46740       -1     
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.22% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/22547/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.74% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/22547/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `97.28% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=footer). Last update [1a798be...77323b8](https://codecov.io/gh/pandas-dev/pandas/pull/22547?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!