BUG: Fix first_last_valid_index, now preserves the frequency.
Checklist

- [X] closes #20499
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I'm not sure if I should add a test somewhere else. Notice that I've added a new function [`_find_first_valid`](https://github.com/mmngreco/pandas/blob/ff8d3964cba04ed08f9f3b0dee3ee5c7514b5ed7/pandas/core/generic.py#L8766-L8785).
can you add a whatsnew note as well @jreback of course, also I forgot asking you where I should add the whatsnew note. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=h1) Report
> Merging [#20569](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d2ab4076512f5571b74e6ea2936910841b10dbe2?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `95.23%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20569/graphs/tree.svg?src=pr&width=650&height=150&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20569      +/-   ##
==========================================
- Coverage   91.84%   91.84%   -0.01%     
==========================================
  Files         152      152              
  Lines       49270    49262       -8     
==========================================
- Hits        45252    45243       -9     
- Misses       4018     4019       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.22% <95.23%> (-0.01%)` | :arrow_down: |
| #single | `41.9% <14.28%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/20569/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.78% <ø> (-0.1%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20569/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.17% <ø> (-0.02%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20569/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.84% <95.23%> (-0.01%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=footer). Last update [d2ab407...48611fa](https://codecov.io/gh/pandas-dev/pandas/pull/20569?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback I added whatsnew note under "Bug fixes: Indexing" section. thanks @mmngreco 