Revert to `@com_google_protobuf` dependency name after `@bazel_tools` deprecation

Introduced in #15362, we are using `@com_google_protobuf` as the repository name for the Protobuf dependency. This was a necessary workaround to support both bzlmod and traditional WORKSPACE-based builds.

The underlying issue is that the `@bazel_tools` repository, which is a "magic" repository built into Bazel, has an internal dependency on Protobuf using the name `@com_google_protobuf`. This reference cannot be overridden using `repo_mapping` in `MODULE.bazel`, forcing us to adopt the `@com_google_protobuf` name for consistency in our WORKSPACE setup.

The Bazel team has [indicated](https://github.com/bazelbuild/bazel/issues/4301#issuecomment-2456643183) that they plan to slowly deprecate `@bazel_tools` and are not planning to make improvements to it. Once `@bazel_tools` is deprecated and the repositories that rely on it have migrated, we should revert our Protobuf dependency to use the canonical `@protobuf` name, as used by bzlmod.

This change will simplify our build configuration and align our project with the broader Bazel ecosystem's conventions.

Reference:
```
diegomarquezp:google-cloud-cpp$ bazelisk query --google_default_credentials --noenable_bzlmod --enable_workspace 'somepath(//google/cloud/storage:google_cloud_cpp_storage_grpc, @com_google_protobuf//src/google/protobuf:arena)'
//google/cloud/storage:google_cloud_cpp_storage_grpc
@googleapis//google/storage/v2:storage_cc_proto
@bazel_tools//tools/proto:cc_toolchain
@com_google_protobuf//:cc_toolchain
@com_google_protobuf//src/google/protobuf:protobuf
@com_google_protobuf//src/google/protobuf:micro_string
@com_google_protobuf//src/google/protobuf:arena
```

Start with the WORKSPACE and MODULE.bazel dependency configuration described in the issue, then run the referenced Bazel query to inspect the current repository paths. This work is blocked on the planned deprecation and migration of @bazel_tools. Done means the configuration can use the canonical @protobuf name while both supported build modes remain consistent.

Written by the indexing model from the issue text.