RFR: 7903201: README.md referes old generated code pattern

[Athijegannathan Sundararajan]

backporting applicable parts of the same fix from jdk19. 

* using latest generated code pattern in README.md
* using module-version in --version output

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Commit messages:
 - 7903201: README.md referes old generated code pattern

Changes: https://git.openjdk.java.net/jextract/pull/43/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=43&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903201
  Stats: 11 lines in 2 files changed: 1 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jextract/pull/43.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/43/head:pull/43

PR: https://git.openjdk.java.net/jextract/pull/43