DOC: 0.24 release date
Also some gitignore cleanup.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24930?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@0bc2d3e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24930/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24930?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #24930   +/-   ##
=========================================
  Coverage          ?   92.38%           
=========================================
  Files             ?      166           
  Lines             ?    52389           
  Branches          ?        0           
=========================================
  Hits              ?    48398           
  Misses            ?     3991           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (?)` | |
| #single | `42.89% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24930?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/24930?src=pr&el=footer). Last update [0bc2d3e...8822c81](https://codecov.io/gh/pandas-dev/pandas/pull/24930?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/24930?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@0bc2d3e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24930/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24930?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #24930   +/-   ##
=========================================
  Coverage          ?   92.38%           
=========================================
  Files             ?      166           
  Lines             ?    52389           
  Branches          ?        0           
=========================================
  Hits              ?    48398           
  Misses            ?     3991           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (?)` | |
| #single | `42.89% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24930?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/24930?src=pr&el=footer). Last update [0bc2d3e...8822c81](https://codecov.io/gh/pandas-dev/pandas/pull/24930?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
