Update DataFrame code in data/

[tlienart]

┌ Warning: `by(d::AbstractDataFrame, cols::Any, f::Pair{<:ColumnIndex}; sort::Bool = false, skipmissing::Bool = false)` is deprecated, use `combine(f, groupby(d, cols, sort = sort, skipmissing = skipmissing))` instead.
│   caller = top-level scope at none:1
└ @ Core none:1
→ evaluating code [...] (data/dataframe.md)                                                       ┌ Warning: `by(d::AbstractDataFrame, cols::Any; sort::Bool = false, skipmissing::Bool = false, f...)` is deprecated, use `combine(groupby(d, cols, sort = sort, skipmissing = skipmissing), [if in_col isa ColumnIndex
│             in_col
│         else
│             AsTable(in_col)
│         end => (fun => out_col) for (out_col, (in_col, fun)) = f]...)` instead.
│   caller = ip:0x0
└ @ Core :-1
→ evaluating code [...] (data/dataframe.md)                                                       ┌ Warning: `aggregate(d, cols, f, sort=false, skipmissing=false)` is deprecated. Instead use combine(groupby(d, cols, sort=false, skipmissing=false), names(d, Not(cols)) .=> f)`
│   caller = top-level scope at none:1