BUG: Merge timezone aware data with DST
- [x] closes #18885
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

When merging timezone aware data using `where`, make the comparison in UTC to avoid relocalizing on an ambiguous time.
Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimelike.py`](https://github.com/mroeschke/pandas/blob/ce784e005f241868d1181afce9e172c1f391d595/pandas/core/indexes/datetimelike.py) !

 - There are no PEP8 issues in the file [`pandas/tests/reshape/merge/test_merge.py`](https://github.com/mroeschke/pandas/blob/ce784e005f241868d1181afce9e172c1f391d595/pandas/tests/reshape/merge/test_merge.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22825?src=pr&el=h1) Report
> Merging [#22825](https://codecov.io/gh/pandas-dev/pandas/pull/22825?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2f1b842119bc4d5242b587b62bde71d8f7ef19f8?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/22825/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22825?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22825      +/-   ##
==========================================
+ Coverage   92.18%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50830    50833       +3     
==========================================
+ Hits        46860    46863       +3     
  Misses       3970     3970
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <100%> (ø)` | :arrow_up: |
| #single | `42.37% <11.11%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22825?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/22825/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.11% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22825?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/22825?src=pr&el=footer). Last update [2f1b842...e4491d8](https://codecov.io/gh/pandas-dev/pandas/pull/22825?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Addressed your comments @jreback and all green. All green besides a unrelated, failed connection to a s3 parquet test.  All green besides a unrelated, failed connection to a s3 parquet test. thanks @mroeschke 