Switch to #f_upcoming_release slack channel

What

This MR removes the usage of monthly release channels (i.e. f_release_13_0) in favor of a single channel #f_upcoming_release

Why

Historically we used the channel separation in order the let the old release managers take care of patch release and let the new release manager take care of the ongoing monthly release.

This is no longer the case, release manager on duty takes care of all the releases, and this separation only adds confusion and monthly chores.

It's not uncommon in the first week of the release cycle to have messages spread across the two channels.