Fix bugs in WeekOfMonth.apply, Week.onOffset
In the process we get rid of `WeekOfMonth.getOffsetOfMonth` and `LastWeekOfMonth.getOffsetOfMonth`, which were idiosyncratic what arguments they passed to `datetime`.

The issues this addresses are orthogonal to #18762, but the code affected does overlap.  In particular, after this, `roll_monthday` will not be needed in 18762.

closes #18864
closes #18672 
closes #18510
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18875?src=pr&el=h1) Report
> Merging [#18875](https://codecov.io/gh/pandas-dev/pandas/pull/18875?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/316acbf065f5d1e30fdc7fcced3a53a2f4b14655?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18875/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18875?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18875      +/-   ##
==========================================
- Coverage   91.65%   91.64%   -0.01%     
==========================================
  Files         154      154              
  Lines       51368    51364       -4     
==========================================
- Hits        47080    47074       -6     
- Misses       4288     4290       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.51% <100%> (-0.01%)` | :arrow_down: |
| #single | `40.84% <21.62%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18875?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/18875/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `96.78% <100%> (-0.17%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18875?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/18875?src=pr&el=footer). Last update [316acbf...e71374f](https://codecov.io/gh/pandas-dev/pandas/pull/18875?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Ping thanks!