BUG: fix astype conversion string -> float
- [x] Is a step towards closing #37626
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Fixes the `string` -> `float` conversion as discussed in #37626 in presence of `pd.NA`. I used the same approach as used in `string` -> `Int64`. Note that differently to the `Int64` case, we return a `numpy.ndarray`, not a `pd.array`.

I added a test that compares `pd.Series`. Replacing this with `pd.array` the `tm.assert_numpy_array_equal` assertion will raise, as `expected` is of type `FloatingArray`. IIUC this is a new feature of pandas 1.2.0. Should we return this in `.astype("float")` by default?