Make update_feeds(scheduled=True) the default

[lemon24]

Already mentioned in https://github.com/lemon24/reader/issues/291, but perhaps it is worth the slight compatibility break[^1] or accelerating 4.0 – there's no point in being able to be polite if you're not polite by default (i.e. reader should do the right thing even if the user doesn't read the documentation).

[^1]: note it's a behavior change, not an API change, so it'd be less bad in some ways