CI: Remove centos-7

[mkoeppe]

EOL 2024-06-30, CI now fails https://github.com/mkoeppe/sage/actions/runs/9970528956/job/27549649531#step:11:66

#6 [with-system-packages 2/4] RUN yum install -y centos-release-scl
#6 0.267 Loaded plugins: fastestmirror, ovl
#6 0.357 Determining fastest mirrors
#6 0.867 Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch=x86_[64](https://github.com/mkoeppe/sage/actions/runs/9970528956/job/27549649531#step:11:65)&repo=os&infra=container error was
#6 0.867 14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"

📝 Checklist

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• I have created tests covering the changes.
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⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit a8e59c6; changes) is ready! 🎉
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[fchapoton]

ok

[mkoeppe]

Thanks.