DEPR: remove Int64Index, UInt64Index, Float64Index from public namespace
Progress towards #42717. In a followup they will be removed internally.
Thanks @topper-123 (very much for tackling this tricky deprecation :) ) Yeah, it has been surprisingly difficult; not having a Windows machine has especially been difficult when debugging int32/int64 issues. Still, I think I got it figured out now and I'm sure tomorrow I'll get #49560 to pass and then the rest is easy.