Revert "playbook: use OBS Debian 9 for all version of Debian <= 9"

This reverts commit 097e1cd0.

This is not working because of dependencies on newer libc6 and
libstdc++6.
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Status Job ID Name Coverage
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passed ERP5.StandaloneDeploymentScriptTest-Debian.Bullseye

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passed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Bullseye

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passed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Bullseye

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passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

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