Refactor/remove obsolete resources and fields

[adrianburusdbt]

This PR removes all attributes and resources that were marked as deprecated in v1, as part of the v2 major release.

Breaking changes
dbtcloud_project_artefacts — resource removed entirely.

dbtcloud_webhook — webhook_id removed from both the resource and data source. Use id instead. The data source id is now required.

dbtcloud_repository — fetch_deploy_key removed from resource and data source.

dbtcloud_databricks_credential — adapter_type and target_name removed from the resource. target_name removed from the data source.

dbtcloud_spark_credential — target_name removed from resource and data source.

dbtcloud_job — top-level timeout_seconds removed from the resource; use execution { timeout_seconds = N } instead. deferring_job_id removed from the singular data source. deferring_job_definition_id removed from the plural data source.

Migration scripts
A migration_scripts_to_v2/ directory has been added with one Python script per affected resource and a migrate_all.py master runner. Each subfolder contains the script, an example input file, and a README. Run a dry-run across your Terraform root to preview all changes before applying:

python migration_scripts_to_v2/migrate_all.py --dry-run ./path/to/terraform/
python migration_scripts_to_v2/migrate_all.py ./path/to/terraform/

Other fixes
Transient HTTP 500s caused by infrastructure errors (identified by a non-JSON Content-Type header) are now retried with exponential backoff in the API client. Application-level 500s are still returned immediately. Request bodies are also now correctly replayed on retries, fixing a bug where retry attempts would send an empty body.