REF: move ExtensionIndex.map to be part of DatetimeLikeArrayMixin.map
Follow-up to #51809.

`ExtensionIndex.map` is only used by the datetime-like indexes (`DatetimeIndex.map` etc.) and contains no index-specific functionality, so shouldn't be an index method.

By moving this method to `DatetimeLikeArrayMixin.map` we simplify the code paths quite a bit when dealing with `DatetimeArray.map` etc. In the next PR I will make  `na_action="ignore"` work as expected and close #51644. 
Thanks @topper-123 