Update samples with stable image tag

[git-hyagi]

Our pipeline was failing for tests with nightly image, but was running fine for tests using stable. API logs:

2024-04-12T18:00:24.335914694Z pulp [e336954360eb4dd7907c1885ece3aca6]: django.request:ERROR: Internal Server Error: /v2/test/fixture/blobs/uploads/
2024-04-12T18:00:24.335953256Z Traceback (most recent call last):
2024-04-12T18:00:24.335959187Z   File "/usr/local/lib/python3.9/site-packages/django/core/handlers/exception.py", line 55, in inner
2024-04-12T18:00:24.335965158Z     response = get_response(request)
...
2024-04-12T18:00:24.336152879Z     raise LookupError("Could not determine ViewSet base name for model {}".format(model_class))
2024-04-12T18:00:24.336158811Z LookupError: Could not determine ViewSet base name for model <class 'pulp_container.app.models.Upload'>

Checking the versions of pulp_container plugin in each installation:

stable
  "container": "2.19.2",
nightly
  "container": "2.20.0.dev",

Updating the samples to use the same image to keep consistency and avoid such "misdirection" where the pipeline failure was not really because of the operator.

[noissue]

Thank you for your contribution!

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• https://github.com/pulp/pulp-operator/issues/new
• https://docs.pulpproject.org/pulpcore/contributing/git.html#commit-message

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