Deprecate creation of legacy package sets

Each package set created by the registry is also mirrored into the old `packages.dhall` format used by the legacy Spago package manager. Now that Spago is available at v1.0.0 we intend to deprecate support for legacy spago. That means we will no longer publish legacy package sets no earlier than **August 1, 2026** — though the exact date isn't set.

Relates to #744.