DistUpgrade: update download rates in estimated download message

Currently, the estimated download time message references 1Mbit DSL and
56k modems. These references are not useful for most modern download
rates. Update the message to reference 5Mbit and 40Mbit rates instead,
using the data in [1] as a guide.

While here, make the computation compliant with [2].

[1] https://www.cable.co.uk/broadband/speed/worldwide-speed-league/
[2] https://wiki.ubuntu.com/UnitsPolicy

LP: #1947581