Remove functionality and artifacts in this repo and point folks to dbt-adapters

[mikealfare]

Problem

We merged dbt-spark into the dbt-adapters monorepo. We want to make sure that users are alerted to look there and we want to avoid accidentally updating this repo.

Solution

• update docs to point to dbt-adapters or remove them completely
• remove irrelevant workflows
• remove things like dependabot
• remove the ability for the community to create issues in this repo and point them to dbt-adapters so that new issues don't get ignored