DEPR: Certain Grouper and Grouping attributes

[rhshadrach]

#55738 reworks some of the internals for groupby, the following attributes are no longer necessary.

• BaseGrouper.group_keys_seq
• BaseGrouper.reconstructed_codes
• Grouping.group_index
• Grouping.result_index
• Grouping.group_arraylike

I'd expect these to be almost exclusively used internally in pandas, but since they are publicly accessible I think we should deprecate these and remove them in 3.0.