build: make warning about rebuilt pkgs a verbose message

Let warnings like the following not get displayed in the regular pmbootstrap output anymore, only in 'pmbootstrap log' if -v / --verbose was used. This message informs the user that a package's dependencies are newer than the package itself. But the WARNING makes it sound like this is something to be concerned about, whereas in reality this is fine. In this example, postmarketos-mkinitfs has gotten a new feature / fix after postmarketos-base and there's no need to rebuild postmarketos-base.

[18:02:59] WARNING: postmarketos-base depends on rebuilt package(s) postmarketos-mkinitfs (use 'pmbootstrap build postmarketos-base --force' if necessary!)