DEPR: Remove Int64Index, UInt64Index, Float64Index and Index._is_backward_compat_public_numeric_index

[topper-123]

In Pandas 2.0 NumericIndex should be the default numerical index and Int64Index, UInt64Index and Float64Index should be removed. See #41153 & #41272 for more discussion.

#41153 adds an internal attribute _is_backward_compat_public_numeric_index to the Index class (and its numeric child classes). This is only used to keep track of whether an index is a NumericIndex or one of its child indexes (Int64Index, UInt64Index and Float64Index) in order to maintain backwards compatability in Pandas 1.x. It will not be needed in pandas 2.0 after the child classes are removed and should removed in pandas 2.0 also.