Introduces a new database config property `seeds` to control whether
seeds are loaded during `db:prepare` which defaults to `true` for
primary database configs and `false otherwise.

Previously, the `db:prepare` task would load seeds whenever a new
database is created, leading to potential loss of data if a database
is added to an existing environment.

Also, update the Active Record Migrations guide to more accurately
describe what the `db:prepare` task does, correcting some
misinformation introduced in #49480 along the way.

Closes #53348