move `loaded_at_field` and `loaded_at_query` under config

[graciegoheen]

Housekeeping

• I am a maintainer of dbt-core

Short description

We're moving freshness under config -> #11506

We should do the same for loaded_at_field and loaded_at_query.

Acceptance criteria

• move loaded_at_query and loaded_at_field to be configs
• issue deprecation warning for setting loaded_at_field as a property (loaded_at_query has not yet been released in core final version, so we shouldn’t need a deprecation warning for it? unless cloud folks are using it on latest release track)

[FishtownBuildBot]

Opened a new issue in dbt-labs/docs.getdbt.com: dbt-labs/docs.getdbt.com#7530