[#38390] Bug fix for Datetime64Formatter with values of ndim > 1
- [x] closes #38390
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This change fixes a bug in `Datetime64Formatter` that errors with values of ndim > 1. This was discovered when using an ExtensionArray with 2D values of datetime64 and attempting to format/display. The formatter currently allows for 2D values, but will end up returning nested strings instead of a flat list of string. The same error occurs when using the `Datetime64Formatter` directly on 2D values.

The fix is to flatten the values to get a flat list of formatted datetime64 strings, then reshape, and then format a second time over the outer dimension to produce the properly nested list format and the final flat list of strings.