Warn about unsupported Python 3.10

[mik-laj]

Related: #19059
Some users are unaware of the supported Python versions and then have trouble using Airflow. It's best to alert them to be aware of potential compatibility issues. For example, see:
https://stackoverflow.com/questions/69615585/unable-to-start-airflow-webserver-with-python-3-10

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^ Add meaningful description above

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[uranusjr]

Not entirely sure if it's best to reference a GitHub issue. We should probably create a documentation page explaining the Python version support policy, and change the message to a more generic Python {version} is not officially supported on this version of Airflow.

[mik-laj]

@uranusjr We can describe the assumptions in general, but that won't give a full description of whether Airflow supports Python 3.10 or not, as Python 3.10 may not be supported by dependency issues that can be fixed after a new version is released. I will also add information to the documentation about the issue.

CDAP also informs users of known issues with a link to the ticket. https://cdap.atlassian.net/wiki/spaces/DOCS/pages/1162707265/CDAP+Release+6.4.0#Known-Issues

[uranusjr]

One nitpick, otherwise lgtm.

[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.

[jedcunningham]

LGTM, just 1 trivial nit.