blubber.yaml: Use blubber/buildkit:v0.24.0
Switch to the newest official Blubber frontend.
Remove blank-requirements.txt since that is no longer needed to
enable the Python builder (and empty requirements list is sufficient).
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Switch to the newest official Blubber frontend.
Remove blank-requirements.txt since that is no longer needed to
enable the Python builder (and empty requirements list is sufficient).