RFC deprecate readsfrom/writesto in favor of simply using open on AbstractCmd

Supersedes #6945.   Now you can simply do e.g.
```julia
open(readline, `ls`, "r")
```
to read from a command or
```julia
open(`less`, "w", STDOUT) do io
    for i = 1:1000
          println(io, i)
    end
end
```
to write to a command.

The deprecated `readsfrom` and `writesto` commands are currently used only in 5 packages: Graphs (cc: @pozorvlak, @lindahua), AWS (cc: @amitmurthy), ClusterManagers (cc: @nlhepler), ImageView (cc: @timholy), and WinRPM (cc: @ihnorton, @vtjnash).

cc: @StefanKarpinski, @IainNZ