docs: deprecate google.cloud.firestore_v1.rate_limiter.utcnow

[Mariatta]

These will be deprecated starting in Python 3.12.

Replaced:

• datetime.datetime.utcnow() with datetime.datetime.now(tz=datetime.timezone.utc)

• datetime.utcfromtimestamp() with datetime.fromtimestamp(tz=datetime.timezone.utc)

Source: python/cpython#103857

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Fixes #<issue_number_goes_here> 🦕

BEGIN_COMMIT_OVERRIDE
fix: Remove used of deprecated datetime.datetime.utcnow() and datetime.utcfromtimestamp()
docs: deprecate google.cloud.firestore_v1.rate_limiter.utcnow
END_COMMIT_OVERRIDE

[pganssle]

Looks good from what I can tell, except for the change to the public interface, which you'd know more about than me.

[meredithslota]

Added do not merge here as we want to verify that we're doing this consistently across any other Python libraries that use this.

[daniel-sanche]

LGTM

[parthea]

I filed #812 for the issue Using the synchronous google.api_core.retry.Retry with asynchronous calls may lead to unexpected results. Please use google.api_core.retry_async.AsyncRetry instead. which appears in the failing presubmit Kokoro Prerelease Dependencies