[Bug] Remove keyfile from BigQueryCredentials._connection_keys

[McKnight-42]

Is this a new bug in dbt-bigquery?

• I believe this is a new bug in dbt-bigquery
• I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

follow-up to dbt-labs/dbt-snowflake#906 + dbt-labs/dbt-snowflake#756

keyfile is included in BigQueryCredentials._connetion_keys, and therefore the Jinja {{ target }} dictionary. While this value doesn't explicitly contain sensitive information itself, I'd put it in the category of password-like fields rather than the category of fields that a user needs to access at runtime.

Expected Behavior

keyfile is not included in _connection_keys

Steps To Reproduce

n/a

Relevant log output

n/a

Environment

- OS:
- Python:
- dbt-core:
- dbt-bigquery:

Additional Context

No response