update business-pay

- ~leave entity pay alone, duplicate, and make changes in ??~
- ~Patrick Wei will instill in CI/CD as these are completed~
- already in GCP I think? Is this for updating it to integrate with the other new services (ie. business-bn)? -*kj*
- clean out old stuff, upgrade the libs, replace OCP integration with GCP integration (may need to have some of this feature flagged at first) - *have a quick chat with thor on this before starting*

QA not in scope for this ticket. QA prep to be tackled in separate ticket.

The issue names no files, tests, or entry points. First locate the business-pay service and confirm with the maintainer what GCP integration and feature-flag scope are intended; compare it with business-bn and review the current OCP integration. Done means obsolete code is removed, libraries are upgraded, and the agreed GCP integration works; QA is explicitly out of scope.

Written by the indexing model from the issue text.