Replace pmaports channels stable, stable-next

Translate the pmaports channels "stable" to "v20.05" and "stable-next" to "v21.03", so these have the same channel name as the pmaports.git branch name.

The original plan was to switch the "stable" channel from the "v20.05" branch to the "v21.03" branch when the release is done. However, now that we are close to that, I'm realizing that this would not be useful. It would lead to conflicts in the dir with locally built packages (default: ~/.local/var/pmbootstrap/packages/$CHANNEL). And it would make it awkward to go back to a previous branch (we may name it old-stable for the time being, but what after that, old-old-stable?).

Related: #2015 (closed)