Remove top-level `freshness` property from model

[aranke]

Housekeeping

• I am a maintainer of dbt-core

Short description

Follow-up from #11644 (comment)

Acceptance criteria

freshness is no longer a top-level property for models

Suggested Tests

Remove and/or fix existing tests

Impact to Other Teams

N/A

Will backports be required?

No

Context

No response

[QMalcolm]

Closign in favor of #11713 because it has a little more detail