RFR: 2481: Update .github/workflows/ci.yml for Linux

[Zhao Song]

Ubuntu-20.04 was deprecated on April 15th, 2025.
https://github.com/actions/runner-images/issues/11101

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Commit messages:
 - SKARA-2481

Changes: https://git.openjdk.org/skara/pull/1715/files
  Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1715&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2481
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1715.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1715/head:pull/1715

PR: https://git.openjdk.org/skara/pull/1715