CASSANDRA-20181: Update cqlsh Docker testing instructions to use public python:slim images on 4.0

[arvindKandpal-ksolves]

What this PR does / Why we need it:

This PR updates the pylib/README.asc to replace the custom Ubuntu Dockerfiles with publicly available python:X.Y-slim images for testing cqlsh. This addresses the reviewer's feedback, making it easier and more accessible for users who have Docker but not Python installed locally, without needing to build custom images.

Changes:

• Replaced custom image build instructions with direct docker run commands using python:slim.
• Included the --net="host" fallback command for Linux users where host.docker.internal is not supported.
• Explicitly documented the lowest and highest supported Python versions for this branch.

patch by Arvind Kandpal; for CASSANDRA-20181

[michaelsembwever]

let's instead keep this, but use publicly available images…
(so it remains useful for folk who have docker but not python installed)

To run cqlsh using publicly available Docker images to connect to Cassandra on the same host, with `$CASSANDRA_DIR` pointing to either the source or install directory where `bin/cqlsh` is located.

For Python 3.8:

  docker run -v $CASSANDRA_DIR:/code -it python:3.8-slim /code/bin/cqlsh host.docker.internal

For Python 3.11:

  docker run -v $CASSANDRA_DIR:/code -it python:3.11-slim /code/bin/cqlsh host.docker.internal

If `host.docker.internal` isn't supported, then you can use `--net="host"`:

  docker run --net="host" -v $CASSANDRA_DIR:/code -it python:3.8-slim /code/bin/cqlsh

This will spawn a cqlsh instance inside the Docker container. It will try to connect to the Cassandra instance running on the Docker host at port 9042. If you have Cassandra running elsewhere, replace host.docker.internal with the IP / hostname as usual.

updating the list^ for the python versions each cassandra branch actually supports (does 4.0 support 3.11 ?)

[arvindKandpal-ksolves]

I have try with the python 3.11 in docker ,it works :
used command :
docker run -v $CASSANDRA_DIR:/code -it python:3.11-slim sh -c "/code/bin/cqlsh host.docker.internal"

[arvindKandpal-ksolves]

Thanks for review, @michaelsembwever ,😃
I will test comments in local and will update all patches accordingly .
And will ensure I will add correct versions in each patch.