stackdriver: upgrade to gcp_auth 0.12

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 53.3%. Comparing base (56a6f45) to head (7eaa4cd). Report is 9 commits behind head on main.

Files	Patch %	Lines
opentelemetry-stackdriver/src/lib.rs	0.0%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #75 +/- ## ======================================= + Coverage 52.3% 53.3% +1.0% ======================================= Files 38 38 Lines 4967 5026 +59 ======================================= + Hits 2598 2680 +82 + Misses 2369 2346 -23 ```

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[djc]

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