Update images used in testing an airgapped CKF 1.9 deployment

[mvlassis]

Closes #1051

Firstly, this PR creates separate directories for testing an airgapped deployment on CKF 1.8 and 1.9, under tests/airgapped/1.8 and tests/airgapped/1.9 respectively.

Additionally updates the manually retrieved images in tests/airgapped/ckf-1.9-testing-images.txt for CKF 1.9.

The images are updated as follows:

• charmedkubeflow/pipelines-runner:ckf-1.8: Updated to charmedkubeflow/pipelines-runner:ckf-1.9 that was created as part of this task.
• docker.io/kubeflowkatib/simple-pbt:v0.16.0: Updated to v.0.17.0 that matches the Knative version used in CKF 1.9.
• ghcr.io/knative/helloworld-go:latest: Kept the same, as it is still the same tag used in this tutorial
• gcr.io/kubeflow-ci/tf-mnist-with-summaries:1.0: Updated to kubeflow/tf-mnist-with-summaries:latest, similar to the images we used for the UATs, as seen here. I confirmed that this image can indeed run with the --fake-data option we give, by running:

  docker run --rm kubeflow/tf-mnist-with-summaries:latest \
  python /var/tf_mnist/mnist_with_summaries.py --fake_data=True

A small addition is changing the default values (Juju 3.4, ckf-1.9-testing-images.txt) in tests/airgapped/setup.sh.

[kimwnasptd]

@mvlassis @NohaIhab now that https://github.com/canonical/bundle-kubeflow/issues/1036 is resolved, should we have dedicated files with the testing images needed for each release into the sub-folders introduced by this PR?

And then they could be used with the --append-images arg of the scripts/get-images.py script

[NohaIhab]

@kimwnasptd agreed, @mvlassis let's have a testing-images.txt in the directory for each release as we are saying in the README