pmaports.cfg: add channel (!1141)

Replace branch_pmos and branch_alpine keys (that were never used) with a
new channel key. pmbootstrap will look up the branches related to a
channel in channels.cfg of the master branch. While at it, modernize the
file.

pmaports.cfg

+# Reference: https://postmarketos.org/pmaports.cfg
 [pmaports]
-# Version numbers (for sanity checks in pmbootstrap)
-# - "version" gets compared with "pmb.config.pmaports_min_version"
-# - "pmbootstrap_min_version" gets compared with "pmb.config.version"
-version=6
+version=7
 pmbootstrap_min_version=1.18.0
-branch_pmos=master
-# Compatible branch from Alpine's aports repository
-branch_alpine=edge
+channel=edge