ENH: IntegerArray.astype(dt64)
- [x] closes #32435
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Let's us de-kludge to_datetime code, getting rid of another _ndarray_values usage.
By my count, after today's slew of values_from_object/internal_get_values PRs, we'll have

- 9 `_ndarray_values` usages
- 3 `values_from_object` usages
- 10 `_internal_get_values` usages
- 1 `Block.to_dense`

(not counting usages in tests, docstrings, or in the each other's implementations) thanks.

can you add a release note in a followon. (I guess this is a bug fix)