tox: remove CentOS 8 from the available testing scenarios (backport #302)

[mergify]

Due to CentOS 8 eol on May 31, 2024*, CentOS 8 is removed as a testing scenario OS

*https://blog.centos.org/2023/04/end-dates-are-coming-for-centos-stream-8-and-centos-linux-7/

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This is an automatic backport of pull request #302 done by Mergify.

[mergify]

Cherry-pick of e57492b has failed:

On branch mergify/bp/pacific/pr-302
Your branch is up to date with 'origin/pacific'.

You are currently cherry-picking commit e57492b.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tox.ini

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally