The engine shipped opt-in behind --new-completion, with the old scripts
still on --completion. That split was never released, so flip it now
rather than carry a third flag through a deprecation later.
--completion serves the engine wrappers; the hand-written scripts move to
completion/legacy/ and stay reachable via --legacy-completion for a
release or two.
.goreleaser.yml needed to follow: it packages the static files from
completion/ into the deb/rpm/apk contents and the Homebrew cask, so
leaving it untouched would have shipped the engine to anyone running
`eval "$(task --completion zsh)"` and the old scripts to everyone
installing from a package. The paths it references are unchanged and now
resolve to the wrappers. Its archive glob is narrowed at the same time,
so the test harness under completion/tests/ stops being shipped in the
release archives.
