RFR: 2035: [GHA] Update checkout action to v4

[Erik Joelsson]

Update the GitHub checkout action to v4 (latest). This was recently done in the JDK repositories.

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Commit messages:
 - SKARA-2035

Changes: https://git.openjdk.org/skara/pull/1557/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1557&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2035
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1557.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1557/head:pull/1557

PR: https://git.openjdk.org/skara/pull/1557