Bundle for 1.9 is missing the branch gh metadata

[kimwnasptd]

Bug Description

This was found while working on #1036

The previous bundles for 1.8 had _github_repo_branch metadata keys in the bundle, in order for the scanning scripts to gather the images from the corresponding branches.
https://github.com/canonical/bundle-kubeflow/blob/main/releases/1.8/stable/kubeflow/bundle.yaml#L11

But for 1.9 everything is set to main. This means that all scripts now are going to be using the tools/get-images.sh scripts from the main branches of the charms

To Reproduce

Run the image gathering script from airgap for the 1.9 bundle. It will end up showing images from the main branches

Environment

Current main
https://github.com/canonical/bundle-kubeflow/tree/b2269189231aea9a1c594def3bd92549ebb3ee0d

Relevant Log Output

bundle: kubernetes
name: kubeflow
docs: https://discourse.charmhub.io/t/3749
applications:
  admission-webhook:
    charm: admission-webhook
    channel: 1.9/stable
    trust: true
    scale: 1
    _github_repo_name: admission-webhook-operator
    _github_repo_branch: main
  argo-controller:
    charm: argo-controller
    channel: 3.4/stable
    trust: true
    scale: 1
    _github_repo_name: argo-operators
    _github_repo_branch: main
...

Additional Context

No response

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-6147.

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