Use pmbootstrap_v3.cfg as config file (MR 2350)

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.

pmb/parse/arguments.py

@@ -818,7 +818,7 @@ def get_parser():
         dest="config",
         type=lambda x: Path(x),
         default=pmb.config.defaults["config"],
-        help="path to pmbootstrap.cfg file (default in ~/.config/)",
+        help="path to pmbootstrap_v3.cfg file (default in ~/.config/)",
     )
     parser.add_argument(
         "-mp",