fix(deps): add upper-bound google-auth dependency

[bbatliner]

For consistency with google-api-core, which specifies
google-auth >= 1.21.1, < 2.0dev as a dependency.

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Fixes #1179 🦕

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[bbatliner]

@googlebot I signed it!

[busunkim96]

LGTM, thank you for the patch!

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[bbatliner]

@busunkim96 do you know why this failed the build? There's a unit test failure on tests/test__auth.py:92 but admittedly I am not familiar with this project, and don't know why this change would cause such a failure.

[parthea]

The build failure is unrelated to this PR. The build fails with pip install google-auth==1.25.0 or greater.

With google-auth==1.25.0, the following test fails.

tests/test__auth.py::TestAuthWithGoogleAuth::test_with_scopes_scoped FAILED  

With google-auth==1.24.0 , the same test passes.

tests/test__auth.py::TestAuthWithGoogleAuth::test_with_scopes_scoped PASSED      

[parthea]

I've created issue #1198 to track the unittest failure since it's unrelated to this PR.

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Merge-on-green attempted to merge your PR for 6 hours, but it was not mergeable because either one of your required status checks failed, one of your required reviews was not approved, or there is a do not merge label. Learn more about your required status checks here: https://help.github.com/en/github/administering-a-repository/enabling-required-status-checks. You can remove and reapply the label to re-run the bot.

[bbatliner]

There was no Kokoro build this time, does that need to be started manually?

Thanks for fixing the other issue and for merging it in here, appreciate it.