use `installer :manual` in 21 Casks

As mentioned elsewhere, `installer` is a very late addition to the DSL (#6660, only available in the latest release 0.45.0).

This PR is the most-disruptive change put forward so far during the DSL transition, as >20 Casks are touched, and the relevant backend support is recent.  However, 20 Casks seems reasonably small, and the `method_missing` warning will guide users to upgrade.