We have decided to let the user generate a pmbootstrap_v3.cfg file
instead of attempting to automatically migrate the file. This way we
don't need to worry about potential migration bugs and users can also
temporarily go back to v2 to compare behavior with v3 without problems.
Remove migration logic related to mirror_alpine and
mirrors_postmarketos, before I add support for using multiple
repositories with these again (as needed for bpo). This reduces
complexity and removes a note about "multiple mirrors are not supported"
that won't be valid anymore.
Using pmbootstrap v3 with the old config can cause problems, for example
when having $WORK in the pmaports dir instead of the actual work path.
This is not supported anymore by v3 to reduce complexity. The format of
how mirrors are stored in the config also has changed.
Use a separate config file, so users can go back from v3 to 2.3.x if
they need to (for figuring out a regression) and so users won't run into
bugs when moving from 2.3.x to v3.
