RFR: 7903636: update jextract sample scripts for --source option removal

[Athijegannathan Sundararajan]

* removing --source option in scripts
* removing binary .class generating scripts (as normal scripts do compilation by javac anyway)
* removing unnecessary --enable-preview option in run scripts.
* updating gradle version to be the latest (8.5) so that we can use JDK 21 for gradle run itself.

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Commit messages:
 - 7903636: update jextract sample scripts for --source option removal

Changes: https://git.openjdk.org/jextract/pull/189/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=189&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903636
  Stats: 224 lines in 60 files changed: 0 ins; 182 del; 42 mod
  Patch: https://git.openjdk.org/jextract/pull/189.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/189/head:pull/189

PR: https://git.openjdk.org/jextract/pull/189