officially deprecate `materialized_view` materialization

[dataders]

more than half of the currently open 11 issues are about materialized views.

all six of them were opened before we shipped materialized_view as a Core materialization.

I propose that:

1. deprecate materialized_view from this repo (or even delete it!)
2. announce that the feature is no longer supported here as it is no longer an experimental feature
3. we close the open issues as "wont_fix", and instead redirect folks to the official implementation and to open tickets on the corresponding adapter repo should they encounter issues and problems

proposed issue response copy

Materialized Views have been supported in dbt since dbt-core 1.7 (November 2023). See dbt docs: materialized views for more information.

The version of materialized views in this experimental repo is is long-overdue for official deprecation -- it has not been meaningfully maintained or improved for over three years (see #11).

If you haven't already, you should try out the official implementation! Should you encounter an issue, you can then open bug or enhancement ticket on either dbt-core or the adapter repo for your current data platform.

For now, I'm going to close this ticket as "wont_do"

### Issues about materialized views
- [ ] #14
- [ ] #20 
- [ ] #22 
- [ ] #28 
- [ ] #37 
- [ ] #38