DEPR: Move Numericindex._convert_slice_indexer & ._maybe_cast_slice_bound
Moves`_convert_slice_indexer` & `_maybe_cast_slice_bound` from `NumericIndex`to `Index` in preparation to remove `NumericIndex` and include numpy int/uint/float64 in the base `Index`.

Each commit contains one method moving each, could be easier to view each method separately in the commit. 

xref #42717.
Updated. thx @topper-123 