ADAP-1128: Migrate dbt-postgres into dbt-adapters

[mikealfare]

Migrate dbt-postgres into this repo using the following shell script and call:

repo=$1
source_branch=$2
target_branch=$3

# create a remote for the legacy adapter repo and fetch the latest commits
git remote remove old || true
git remote add old https://github.com/dbt-labs/$repo.git
git fetch old

# merge the updated branch from the legacy repo into the dbt-adapters repo
git checkout $target_branch
git merge old/$source_branch --allow-unrelated-histories

# remove the remote that was created by this process
git remote remove old || true

source ./scripts/migrate-adapter.sh dbt-postgres monorepo/stage monorepo/migrate-dbt-postgres

Update workflows to include dbt-postgres.

Make updates related with code quality checks due to slightly different config.

Merge this PR as a regular merge instead of a squash merge in order to retain git blame history.

[mikealfare]

Integration tests run against this branch: https://github.com/dbt-labs/dbt-adapters/actions/runs/12920407631/job/36032667661

[mikealfare]

The dbt-redshift failures are all the IAM Role env variable issue and the dbt-athena failures are the iceberg retries issue. These are known issues and unrelated to dbt-postgres.