CI: ubuntu.yml: Add GitHub Actions s390x case

Add the s390x case using GitHub Actions ppc64le/s390x service.
https://github.com/IBM/actionspz

We can run the ppc64le/s390x cases only in the registered upstream repositories.
https://github.com/IBM/actionspz/blob/main/docs/FAQ.md#what-about-forked-repos

The following matrix upstream logic is to skip the ppc64le/s390x in the
downstream (fork) repositories.

```
+        upstream:
+          - ${{ github.repository == 'ruby/ruby' }}
```

Use the "os" list to determine the excluded ppc64le/s390x cases by using the
"exclude" syntax. Because the "exclude" syntax are executed before the
"include" syntax.

Add the ubuntu-24.04-ppc64le as a comment, because the GitHub Actions ppc64le
case has the following test errors and failures.
https://bugs.ruby-lang.org/issues/21534

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