channels.cfg: adjust to latest reference

Remove mirrordir_pmos and change stable branch name to v20.05.

Related: https://gitlab.com/postmarketOS/pmbootstrap/-/merge_requests/1912#note_327544608

channels.cfg

@@ -6,12 +6,10 @@ recommended=edge
 description=Rolling release channel
 branch_pmaports=master
 branch_aports=master
-mirrordir_pmos=master
 mirrordir_alpine=edge
 
 [stable]
 description=Upcoming beta release (WIP, DO NOT USE!)
-branch_pmaports=release/20.04
+branch_pmaports=v20.05
 branch_aports=3.11-stable
-mirrordir_pmos=v20.04
 mirrordir_alpine=v3.11