RFR: 1943: Update .github/workflows/ci.yml to fix GHA

[Zhao Song]

Since last month, the GitHub Action for running tests on macOS has been failing consistently, and the test job always failed due to time out. It seems like that GitHub no longer supports tests on macOS-10.15, so we need to update the configuration.

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Commit messages:
 - SKARA-1943

Changes: https://git.openjdk.org/skara/pull/1530/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1530&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1943
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1530.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1530/head:pull/1530

PR: https://git.openjdk.org/skara/pull/1530