RFR: 7903399: sync panama branch of jextract for jdk 21 [v2]

[Athijegannathan Sundararajan]

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

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last revision:

  changed jdk20_home references to jdk21_home in README.md (review comment)

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Changes:
  - all: https://git.openjdk.org/jextract/pull/97/files
  - new: https://git.openjdk.org/jextract/pull/97/files/ba7ed4d8..d893d4a5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=97&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=97&range=00-01

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