Move tz cleanup whatsnew entries to v0.24
Pre-req for #21612. https://github.com/pandas-dev/pandas/pull/21612#pullrequestreview-131551269

Moving tz cleanup whatsnew entries from 0.23.2 to 0.24.0 added in #21491

cc @jreback 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21631?src=pr&el=h1) Report
> Merging [#21631](https://codecov.io/gh/pandas-dev/pandas/pull/21631?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1033e8b1195d4071253889ada60523832285354c?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/21631/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21631?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21631   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files         153      153           
  Lines       49532    49532           
=======================================
  Hits        45525    45525           
  Misses       4007     4007
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |


------

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