RFR: Update GHA to jdk20 [v6]

[Maurizio Cimadamore]

> This PR updates the github actions on the jdk20 branch

Maurizio Cimadamore has updated the pull request incrementally with four additional commits since the last revision:

 - Merge pull request #2 from JornVernee/MAC
   
   Get build working on Mac as well
 - use JDK 17 for Gradle
 - use JDK 19 for Gradle
 - Add mac

-------------

Changes:
  - all: https://git.openjdk.org/jextract/pull/96/files
  - new: https://git.openjdk.org/jextract/pull/96/files/b40c8a5c..6a6fd8c5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=05
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=04-05

  Stats: 9 lines in 1 file changed: 8 ins; 0 del; 1 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