DOC: fix warnings in whatsnew about MI.labels -> codes rename
Follow-up on https://github.com/pandas-dev/pandas/pull/23752 to get rid of 2 warnings.
thanks @jorisvandenbossche  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=h1) Report
> Merging [#24150](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/809c0f4be9a361f599e1c6aa0fb2d6afdd655102?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24150/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24150   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         162      162           
  Lines       51700    51700           
=======================================
  Hits        47671    47671           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24150?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/24150?src=pr&el=footer). Last update [809c0f4...65cb71d](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=h1) Report
> Merging [#24150](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/809c0f4be9a361f599e1c6aa0fb2d6afdd655102?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24150/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24150   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
=======================================
  Files         162      162           
  Lines       51700    51700           
=======================================
  Hits        47671    47671           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `43.02% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24150?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/24150?src=pr&el=footer). Last update [809c0f4...65cb71d](https://codecov.io/gh/pandas-dev/pandas/pull/24150?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
