RFR: 7903399: sync panama branch of jextract for jdk 21

[Athijegannathan Sundararajan]

changed to use --enable-preview --source=21 everywhere along apart from changes done in jdk20 branch

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Commit messages:
 - 7903399: sync panama branch of jextract for jdk 21

Changes: https://git.openjdk.org/jextract/pull/97/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=97&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903399
  Stats: 5292 lines in 124 files changed: 3391 ins; 43 del; 1858 mod
  Patch: https://git.openjdk.org/jextract/pull/97.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/97/head:pull/97

PR: https://git.openjdk.org/jextract/pull/97