DEPR: deprecate default of keep_tz=False of DatetimeIndex.to_series
Closes #17832


Hello @jorisvandenbossche! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/jorisvandenbossche/pandas/blob/75291853c5c39af9b8fef54d4a86919300e901a1/pandas/core/indexes/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_alter_axes.py`](https://github.com/jorisvandenbossche/pandas/blob/75291853c5c39af9b8fef54d4a86919300e901a1/pandas/tests/frame/test_alter_axes.py) !

 lgtm. can you add to the deprecation issue #6581  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23739?src=pr&el=h1) Report
> Merging [#23739](https://codecov.io/gh/pandas-dev/pandas/pull/23739?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e98032db44c95325edec23292be84cebed9af867?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23739/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23739?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23739      +/-   ##
==========================================
+ Coverage   92.25%   92.25%   +<.01%     
==========================================
  Files         161      161              
  Lines       51383    51388       +5     
==========================================
+ Hits        47404    47409       +5     
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (ø)` | :arrow_up: |
| #single | `42.31% <16.66%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23739?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23739/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.14% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/23739/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.55% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23739?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/23739?src=pr&el=footer). Last update [e98032d...d1390ed](https://codecov.io/gh/pandas-dev/pandas/pull/23739?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jorisvandenbossche  I'm looking at enforcing this deprecation now.  Should we just change the default or go ahead and rip out keep_tz altogether?