Enable deprecation: materialization-return

[emmyoop]

Introduced in: v0.15
In the last week, raised by: 14 projects (<1%)

https:/dbt-labs/dbt/blob/96083dcaf51ab7725f1b554891581e1522e32d7d/core/dbt/task/run.py#L208-L225

• Remove the conditional branch if isinstance(result, str) that raises a deprecation warning and handles the string result. If result isn't a dictionary that passes _validate_materialization_relations_dict, we should just raise an exception.
• Remove MaterializationReturnDeprecation from the list of active deprecations

Originally posted by @jtcohen6 in #2798 (comment)