[Toaster] Supported releases list for 2.0

[Barros Pena, Belen]

Our initial plan with Toaster was supporting the master branch plus 2
releases. In 1.8 we supported dizzy and fido. If we stick to the plan, in
2.0 we would drop dizzy and add jethro.

Is this still the plan? We need to update the toasterconf.json files but,
most importantly, the manual as well, so the technical writer needs to
know. If nobody shouts to the contrary, I will assume this is what we'll
do.

Thanks!

Belén