Add tests for "No downgrade policy"

[blakeli0]

All dependencies of the Cloud SDK should come from the 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 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 deps: Update first-party-dependencies to 3.55.0-rc1 #7332.

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

Possible solutions:

• BomContentTest 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.