fix: add google-auth as a direct dependency

[mukund-ananthu]

1. One of the issues this solves is the test coverage failures in https://github.com/googleapis/python-pubsub/actions/runs/7836080724/job/21382924522?pr=1064
2. pulling the google-auth dependency transitively through google-api-core so we decided to use the same google-auth version as google-api-core which is 2.14.1
   https://github.com/googleapis/python-api-core/blob/82c31184168d92a88ea570780082ab1176cfed53/setup.py#L34
3. Reference PR: fix: add google-auth as a direct dependency python-bigquery-storage#740

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Fixes #<issue_number_goes_here> 🦕

[michaelpri10]

< 3.0.dev0 to match https://github.com/googleapis/python-api-core/blob/82c31184168d92a88ea570780082ab1176cfed53/setup.py#L34

[mukund-ananthu]

Existing change seems to be consistent with googleapis/python-bigquery-storage@56e8958 which also refers to https://github.com/googleapis/python-api-core/blob/82c31184168d92a88ea570780082ab1176cfed53/setup.py#L34 and the way the versions are mentioned for the other lines in the file.

Tagging @parthea to get his input on this.