RFR: 7903436: Changes needed for jdk20-based makefile build

[Jorn Vernee]

These are the changes for jdk20-based makefile builds. Just 2 simple version bumps

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Commit messages:
 - update Java version in makefiles for jdk20

Changes: https://git.openjdk.org/jextract/pull/107/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=107&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903436
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jextract/pull/107.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/107/head:pull/107

PR: https://git.openjdk.org/jextract/pull/107