DOC: move whatsnew on performance from 0.23.2 to 0.24.0
I did not backport the performance improvements in my backporting PRs for now, so this moves that section to 0.24.0.txt. 
(my rationale: let's be conservative for bug fix releases, performance improvements (if they are not fixing severe performance regressions) are not strictly needed in a bug fix release).


Agreed with being conservative in bugfix releases. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21709?src=pr&el=h1) Report
> Merging [#21709](https://codecov.io/gh/pandas-dev/pandas/pull/21709?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ad76ffcca0d92c3885c279c80701c2f4a3f3f177?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/21709/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21709?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21709   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         154      154           
  Lines       49659    49659           
=======================================
  Hits        45640    45640           
  Misses       4019     4019
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <ø> (ø)` | :arrow_up: |
| #single | `41.89% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21709?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/21709?src=pr&el=footer). Last update [ad76ffc...0adcf07](https://codecov.io/gh/pandas-dev/pandas/pull/21709?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 did you move the fixes? or rather not backport them? Yes, I simply did not backport them