Rename release channels to the branch names, except for edge

I'm preparing the upcoming v21.03 release. This is just the second release, so the process for switching from one release to the next one is not established yet.

Right now, we have the following channels:

Channel	Branch
edge	master
stable	v20.05
stable-next	v21.03

The channels are used here:

• channels.cfg of the master branch
• pmaports.cfg of each branch
• ~/.local/var/pmbootstrap/packages/$CHANNEL/

Notably, the channels are not used in the postmarketOS mirror dirs. We simply use the branch names there.

The plan was to switch stable to v21.03 when it is ready.

I'm realizing now that this won't be useful. This causes packages built by pmbootstrap against v20.05 to be used on the v21.03 branch after we make the switch, which makes no sense. (A simple pmbootstrap zap -p would work around this, but still.)

Furthermore, the "stable channel" term is confusing here. v20.05 is the first beta, v21.03 is the next beta, only after that the first real stable version v21.xx is planned.

To resolve all this, I plan the following:

TODO:

• get pmbootstrap MR merged to implement the automatic rename (!2032 (merged))
• make pmbootstrap release at some point (1.30.0)
• some time later, adjust channels.cfg and pmaports.cfg in pmaports.git and require a newer pmbootstrap version. It's not urgent, the old names already get translated automatically, and by not requiring a pmbootstrap version right after it is released, we give users some time to upgrade.