Enable deprecation: adapter-macro

[emmyoop]

adapter-macro

Introduced in: v0.18
In the last week, raised by: 203 projects (~3%)

Pretty simple: we deprecated the adapter_macro context member in favor of adapter.dispatch, and the former currently serves as a passthrough to the latter:

https://github.com/dbt-labs/dbt/blob/484517416f3ec7affb22f3f1be4f19b2ef825332/core/dbt/context/providers.py#L1152-L1212

Let's keep adapter_macro, but change it to immediately raise an exception that encourages users to switch it out for adapter.dispatch instead (perhaps with a link to https://docs.getdbt.com/reference/dbt-jinja-functions/dispatch). Then, we can remove adapter-macro from the list of active deprecations.

Originally posted by @jtcohen6 in #2798 (comment)