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?).
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