RFR: 7903201: README.md referes to old API and old generated code pattern

[Athijegannathan Sundararajan]

* fixed README.md for new API usage and new generated code pattern
* --version uses module version rather than java.version
* fixed makefile --module-version to be 19

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

Changes: https://git.openjdk.java.net/jextract/pull/42/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=42&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903201
  Stats: 17 lines in 3 files changed: 1 ins; 2 del; 14 mod
  Patch: https://git.openjdk.java.net/jextract/pull/42.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/42/head:pull/42

PR: https://git.openjdk.java.net/jextract/pull/42