Remove :appcast and rename :cask_url to :url.

[reitermarkus]

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livecheck is a replacement for appcast, so :appcast should never be needed. The :cask_url naming is weird, since the url is always the cask url. url :url may look weird, but at least is consistent with url :homepage.

[BrewTestBot]

Review period will end on 2020-12-25 at 23:52:58 UTC.

[BrewTestBot]

Review period ended.

[reitermarkus]

Marking as critical so we can start using :url in casks as soon as possible.

[samford]

The only other place we use appcast in livecheck is Livecheck#checkable_urls (which collects URLs in a formula/cask to try when there isn't a livecheck block) but we should keep that until appcast is totally gone from casks.