RFR: Update GHA to jdk20 [v2]

[Maurizio Cimadamore]

> This PR updates the github actions on the jdk20 branch

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Revert to JDK 18

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Changes:
  - all: https://git.openjdk.org/jextract/pull/96/files
  - new: https://git.openjdk.org/jextract/pull/96/files/020d0304..d62b61c0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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