[java-cloud-bom] Add tests for "No downgrade policy"

All dependencies of the Cloud SDK should come from the [shared-dependencies](https://github.com/googleapis/sdk-platform-java/tree/main/java-shared-dependencies). The libraires-bom only aggregates them together and makes sure all client libraries use the same versions of shared-dependencies.

However, the libraries-bom may accidentally downgrade some dependencies. For example,

- Spanner upgraded grpc-gcp to 1.9.0 but shared-dependencies still uses 1.7.0. We had to [explicitly override](https://github.com/googleapis/java-cloud-bom/pull/7308) grpc-gcp version in the libraries-bom. This is a short term solution and we will remove it in the next release cycle.
- For the RC release, we didn't merge the first-party-dependencies upgrade and caused enforcer errors. This is fixed in https://github.com/googleapis/java-cloud-bom/pull/7332.

We need to add tests to make sure no dependencies are downgraded.

Possible solutions:

- [BomContentTest](https://github.com/googleapis/java-cloud-bom/blame/main/tests/src/test/java/com/google/cloud/BomContentTest.java#L69) is already testing some convergency, I was expecting it to fail but it didn't. We might be able to extend the tests for "No downgrade rule".
- Create a test app for each and every handwritten libraries, run Maven enforcer plugin before release.