Re-enable the dependency CI and protobuf CI checks

[lqiu96]

Migrate these two sdk-platform-java jobs over to the monorepo:

• https://github.com/googleapis/sdk-platform-java/actions/workflows/downstream_protobuf_compatibility_check_nightly.yaml
• https://github.com/googleapis/sdk-platform-java/actions/workflows/dependency_compatibility_test.yaml

Ideally, these should be run against the unit tests in these repos :

1. Showcase (GAPIC Coverage)
2. Handwritten (Veneer Coverage)

To ensure that there are no source incompatibilities.

Stretch:

• See if we can also test against handwritten ITs
• If we can test with all the handwritten libraries, we can provide an official dependency compatibility matrix for the major dependencies that we ship in the SDK

EDIT:
July 23 - Added a new stretch goal

[lqiu96]

Actually, it looks like we may have migrated the dependency_compatibility_test into the monorepo:

https://github.com/googleapis/google-cloud-java/blob/1789e011b4cd2a181fac93b476841a2e7270af85/.github/workflows/sdk-platform-java-dependency_compatibility_test.yaml

But may be worth expanding this to also cover the handwritten libraries unit tests