dataflow (fix): pin in to ubuntu:impish (#7823)
dataflow (fix): pin in to ubuntu:impish (#7823)

It looks like on the newly released `ubuntu:jammy` which is what `ubuntu:latest` points to now, the available Python installation is Python 3.10. Even though Apache Beam is available in Python 3.10 through `pip`, there is no "official" support for it so there is no `apache/beam_python3.10_sdk` image.

So we're pinning to `ubuntu:impish` which is the latest ubuntu image with Python 3.9 support.

Co-authored-by: Maciej Strzelczyk <73f0e2c4a9187016816257efb7b44533c71fa84b@google.com>
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples