RFR: Update GHA to jdk20

[Maurizio Cimadamore]

This PR updates the github actions on the jdk20 branch

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Commit messages:
 - Update GHA to jdk20

Changes: https://git.openjdk.org/jextract/pull/96/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=00
  Stats: 15 lines in 1 file changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jextract/pull/96.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/96/head:pull/96

PR: https://git.openjdk.org/jextract/pull/96