gh-38179: `tox.ini`, `.github/workflows`, `.devcontainer`: Remove centos-stream-8 (EOL)
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This platform reached its EOL on 2024-05-31.
https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-
stream-8-and-centos-linux-7/
It can no longer be tested, as seen in
https://github.com/sagemath/sage/actions/runs/9435864412/job/25989853211
We remove it.
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38179
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee
