RFR: Update GHA to jdk20 [v3]

[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:

  Drop JDK 18

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

Changes:
  - all: https://git.openjdk.org/jextract/pull/96/files
  - new: https://git.openjdk.org/jextract/pull/96/files/d62b61c0..54ce1a4e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=96&range=01-02

  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 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