ENH: GH12034 RangeIndex.__floordiv__ returns RangeIndex if possible
xref #12034

Not sure if I should always return `RangeIndex` when length < 2. 
 @kawochen I had the same thoughts for `n=0/1` for the default constructor for a `NDFrame`, but thought its more consistent to always get a `RangeIndex`. Ideally would do the same if at all possible.
 pls add this issue number to the whatsnew note as well.
 ping when green
 updated
 merged via https://github.com/pydata/pandas/commit/8e2936172bdab24d6f5ad9722ec7bd0f249c2d6d

thanks!
