[Feature] Remove dbt-postgres and dbt-tests-adapter from dbt-core

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Now that dbt-postgres has been setup and dbt-test-adapters is in dbt-adapters we want to remove those packages from dbt-core.

For dbt-tests-adapters we will migrate what is in dbt-core to be a test fixtures in the tests/ directory.

This will also involve update release workflows to stop releasing these packages to pypi.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response