DEPR: Deprecated Index.to_datetime
Deprecates both `Index.to_datetime` and `DatetimeIndex.to_datetime` in favor of `pd.to_datetime`.

Partially addresses #8254.

Closes #5612 because `pd.to_datetime` does have a `format` argument.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14096?src=pr) is 85.27% (diff: 100%)

> Merging [#14096](https://codecov.io/gh/pydata/pandas/pull/14096?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14096   diff @@
==========================================
  Files           139        139          
  Lines         50489      50490     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43052      43053     +1   
  Misses         7437       7437          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [0e61847...efa9885](https://codecov.io/gh/pydata/pandas/compare/0e61847e111a3ba181f16c8b9b974c74d360ad2e...efa9885ec0028e95f45c29f00e9c7d5e2258672e?src=pr)
 @jorisvandenbossche : Since Travis is passing, and if there are no complaints, IMO this can be merged?
 I would add the deprecation of Timestamp.to_datetime as well, if you are ok with that?
 The reasons for deprecation are slightly different though.  When I looked into it, the deprecation was a little more complicated.  This change, however, was relatively trivial. :smile:

I would feel more comfortable doing it in a separate PR if that's okay.
 OK, that's fine :-) Thanks!
 @jorisvandenbossche : Also, add that deprecation to #6581.
 @gfyoung yep, thanks, added
