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GH-34595: [C++] Update google-cloud-cpp to v2.8.0 (#34707)

Newer versions of `google-cloud-cpp` include support for workload identity federation (aka BYOID, aka federated identity). There are many other features and improvements too.  FWIW, this library increased its major version (from 1.x to 2.x) simply because it dropped support for C++11. Since Arrow already requires C++17, this is a non-issue.

### Rationale for this change

This fixes #34595.  Newer versions of `google-cloud-cpp` support federated identity (and other features).

### What changes are included in this PR?

### Are these changes tested?

N/A. The existing tests should cover this.

### Are there any user-facing changes?

No.  There are no public facing changes, nor breaks to public APIs.

* Closes: #34595

Authored-by: Carlos O'Ryan <coryan@google.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>