Detect when upstream GAPIC modules may cause issue in Spring

Previous examples of issues:

1. google-cloud-debugger-client was removed and was causing issues in Spring: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/3772
2. Spanner changes are incompatible with Spring: https://github.com/GoogleCloudPlatform/spring-cloud-gcp/pull/3776

Both examples of changes from upstream end up breaking something in Spring (downstream). We weren't aware of these changes breaking anything until upstream had been released.

Can this be improved so that the team is aware of upstream changes that may end up breaking downstream functionality?

Options:
- Downstream testing in each of the upstream repos (HW libraries + google-cloud-java) against spring?
- (For removed modules) Release Manager who removes a GAPIC must also remove the spring module as well.

Start by reviewing the two referenced Spring Cloud GCP pull requests for the debugger-client removal and Spanner incompatibility, then inspect how upstream GAPIC and Google Cloud Java changes are released. Done means the team has an agreed, repeatable way to detect or communicate upstream changes that could break Spring downstream functionality.

Written by the indexing model from the issue text.