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