DEPR: Int64Index, UInt64Index & Float64Index
* [x] xref #42717
* [x] xref #41153 & #42706

Deprecates Int64Index, UInt64Index & Float64Index.

I don't think it will be sensible to emit a deprecation warning for these because they have a lot of internal usage as is, so I've just added texts explaining that these are deprecated and will be removed in Pandas 2.0.

EDIT: it does now emit an deprecation warning when accessed from the main namespace. See changes to `pandas.__init__.py` for implementation.

```python
>>> pd.Int64Index
FutureWarning: pandas.Int64Index is deprecated and will be removed from pandas in a future version. Use pandas.NumericIndex with the appropriate dtype instead.
```

xref #43002 IMHO, we probably should emit some sort of warning to users. I'm a little worried since 2.0 should be the next release after 1.4, and this is a somewhat big change. We can always suppress the deprecation warnings internally. I've made a new version where accessing ``Int64Index`` etc. in the main namespace emits a FutureWarning. So these give warnings now:

```python
>>> import pandas as pd
>>> idx = pd.Int64Index([8, 9])
FutureWarning: pandas.Int64Index is deprecated and will be removed from pandas in a future version. Use pandas.NumericIndex with the appropriate dtype instead.
>>> isinstance(idx, pd.Int64Index)
FutureWarning: pandas.Int64Index is deprecated and will be removed from pandas in a future version. Use pandas.NumericIndex with the appropriate dtype instead.
True
```

but this does not:

```python
>>> idx = pd.Index([8, 9])
>>> idx
Int64Index([8, 9], dtype='int64')
>>> isinstance(idx, pd.NumericIndex)
True
```

This last example doesn’t give warnings, because this access pattern is not deprecated, but will only change meaning in Pandas 2.0 (will return a NumericIndex rather than a Int64Index, + Int64Index is a subclass of NumericIndex).

These warnings should  IMO give users a good hint on where they need to update their code to stay compatible in Pandas 2.0.

 Got some failures still, but will work on them. The idea is completed, so commentd welcome. Updated. Gentle ping… thanks @topper-123 very  nice! @topper-123 want to enforce this deprecation and rip out some code?