Allow google-auth < 3 and google-api-core < 3

[busunkim96]

google-auth recently published a 2.0.0 release which removed support for Python 2.7. google-auth now requires Python >=3.6. No other breaking changes were made. You can see the full list of changes here.

I am opening PRs to expand google-auth version ranges for packages that meet either of the following criteria:

• Package has >10,000 monthly downloads as of June 2021
• Package is owned by a Google team

google-auth is a dependency of many different libraries that interact with Google APIs. Increasing the time and number of packages with compatible pins on google-auth lowers the chance end developers who use multiple libraries will see dependency conflicts.

If possible, please do not require google-auth>=2.0.0 or google-api-core>=2.0.0 until googleapis/google-cloud-python#10566 is resolved, as that will further reduce the likelihood of diamond dependency conflicts.

Googlers, see this doc for more information.

Given the number of google-cloud-* packages this library pins directly I also bumped the pin on google-api-core to < 3.

Constraints files: I looked at https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst#pinned-constraint-files but am not quite sure what that means for this change.

[boring-cyborg]

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!